Adenosine, caffeine, and sleep–wake regulation: state of the science and perspectives

“In the conceptual framework of the two-process model, EEG slow-waves in NREM sleep are thought to reflect sleep homeostatic mechanisms (Borbely, 1982; Borbely et al., 2016; Daan et al., 1984).”

“Adenosine is thought to increase in the extracellular space in the course of prolonged waking, and has been proposed as a homeostatic accumulator of the need to sleep (Benington & Heller, 1995; Brown et al., 2012; Landolt, 2008).”

“Furthermore, chronic caffeine administration increased the number of adenosine receptors in rats and mice (Boulenger et al., 1983; reviewed by Nehlig et al., 1992).”

“…receptors have long been recognised as possible therapeutic targets for the pharmacological treatment of sleep disturbances (Chen et al., 2013; Jacobson & Gao, 2006), no approved drugs to ameliorate disturbed wakefulness and sleep by specifically targeting the adenosine neuromodulatory system…”

“In the conceptual framework of the two-process model, EEG slow-waves in NREM sleep are thought to reflect sleep homeostatic mechanisms (Borbely, 1982; Borbely et al., 2016; Daan et al., 1984).”

“…came from the observations that extracellular adenosine levels in different brain areas in rats and cats tended to increase during wakefulness, decrease during sleep, and to be enhanced after prolonged wakefulness (Huston et al., 1996; Porkka-Heiskanen et al., 1997; Porkka-Heiskanen et al., 2000).”

“Although adenosine receptors have long been recognised as possible therapeutic targets for the pharmacological treatment of sleep disturbances (Chen et al., 2013; Jacobson & Gao, 2006), no approved drugs to ameliorate disturbed wakefulness and sleep by specifically targeting the adenosine…”

“…carrying a conditional, dominantnegative form of the SNARE (dnSNARE) protein that controls adenosine release from activated astrocytes, demonstrated a sleep–wake phenotype that was remarkably reminiscent of conditional A1 receptor knock-out animals (Bjorness et al., 2009; Halassa et al., 2009).”

“Notwithstanding, large genome-wide association studies failed to provide evidence for enriched variant adenosinergic genes in patients with insomnia (Jansen et al., 2019; Van Someren, 2021).”

“Adenosine is thought to increase in the extracellular space in the course of prolonged waking, and has been proposed as a homeostatic accumulator of the need to sleep (Benington & Heller, 1995; Brown et al., 2012; Landolt, 2008).”

“As mentioned above, deamination of adenosine to inosine by ADA occurs primarily under conditions with elevated adenosine levels (Borea et al., 2018).”

“There is evidence in the animal domain that caffeine's main metabolite paraxanthine has similar affinity as caffeine (Snyder et al., 1981) to both receptors (Chou & Vickroy, 2003) and disturbs NREM sleep (Okuro et al., 2010).”

“Work in mice provided strong evidence that caffeine promotes wakefulness primarily by blocking the A2A subtype of adenosine receptors (Huang et al., 2005).”

“The available studies reported either no influence on, stimulation or inhibition of caffeine pharmacokinetic measures by chronic caffeine intake (Nehlig, 2018).”

“The accumulation of adenosine, but also other sleep promoting substances (Krueger et al., 2008), may therefore reduce the activity of wake promoting areas and, in this way, disinhibit sleep promoting areas.”

“…al., 2001; Dagan & Doljansky, 2006; Hansen et al., 2019; Killgore et al., 2006; Killgore et al., 2012; Killgore & Kamimori, 2020; Kohler et al., 2006; Landolt et al., 2004; Lieberman et al., 2002; Paech et al., 2016; Patat et al., 2000; Penetar et al., 1993; Wright, Badia, Myers, & Plenzler, 1997).”

“These compounds lack agonistic activity per se but may amplify the action of endogenous ligands at their receptors by stabilising the ligand–receptor complex (Ferre et al., 2014).”

“These molecules are required for sleep–wake related changes in synaptic plasticity in vitro and the weakening of synapses during sleep in mice (de Vivo et al., 2017; Diering et al., 2017; Hu et al., 2010).”

“Light information reaches the SCN through the retino-hypothalamic tract (Vansteensel et al., 2008), which releases glutamate at its nerve terminals (Ding et al., 1994; Johnson et al., 1988).”

“Notwithstanding, large genome-wide association studies failed to provide evidence for enriched variant adenosinergic genes in patients with insomnia (Jansen et al., 2019; Van Someren, 2021).”

“These molecules are required for sleep–wake related changes in synaptic plasticity in vitro and the weakening of synapses during sleep in mice (de Vivo et al., 2017; Diering et al., 2017; Hu et al., 2010).”

“These molecules are required for sleep–wake related changes in synaptic plasticity in vitro and the weakening of synapses during sleep in mice (de Vivo et al., 2017; Diering et al., 2017; Hu et al., 2010).”

“The functional connectivity within these networks is altered in certain patients with anxiety disorders and insomnia (Chellappa & Aeschbach, 2022).”

“Nevertheless, epidemiological studies support the idea that increased caffeine use is associated with less sleep or more disturbed sleep, also when caffeine is used chronically (Clark & Landolt, 2017; Roehrs & Roth, 2008).”

“Nevertheless, it is interesting to note that one of the most abundant components in the native cannabis plant, cannabidiol, inhibits the ENT1 (Carrier et al., 2006).”

“…came from the observations that extracellular adenosine levels in different brain areas in rats and cats tended to increase during wakefulness, decrease during sleep, and to be enhanced after prolonged wakefulness (Huston et al., 1996; Porkka-Heiskanen et al., 1997; Porkka-Heiskanen et al., 2000).”

“The A2A receptor was thus proposed to provide a promising target as add-on treatment to ameliorate motor and non-motor symptoms such as fatigue in Parkinson's disease (Schwarzschild et al., 2006).”

“Nevertheless, epidemiological studies support the idea that increased caffeine use is associated with less sleep or more disturbed sleep, also when caffeine is used chronically (Clark & Landolt, 2017; Roehrs & Roth, 2008).”

“…Homer1a is a core brain molecular correlate of sleep loss, responds to waking-induced increased extracellular adenosine levels, enhances mGluR5 signalling, and uncouples mGluR5 from the downstream signalling cascades during sleep (Holz et al., 2019; Maret et al., 2007; Martin et al., 2019).”

“Moreover, healthy people with polymorphisms in the ADORA2A gene exhibit elevated risk of anxiety symptoms when consuming normal amounts of caffeine-containing products (Alsene et al., 2003).”

“Subsequent experiments revealed that specifically the genetic deletion of A2A receptors in the shell of the nucleus accumbens blocked caffeine-induced wakefulness (Lazarus et al., 2011).”

“Light information reaches the SCN through the retino-hypothalamic tract (Vansteensel et al., 2008), which releases glutamate at its nerve terminals (Ding et al., 1994; Johnson et al., 1988).”

“The activity increases during waking and REM sleep and decreases during NREM sleep (Deboer et al., 2003) The ultradian changes across the vigilance states are visible on top of the circadian modulation of SCN neuronal activity.”

“The changes in EEG SWA are relatively slow, involving many hours of slow increase during the main waking period and an almost equally slow decreasing process during the main sleep period, independent of the species investigated (Deboer, 2015; Huber et al., 2000).”

“Adenosine is thought to increase in the extracellular space in the course of prolonged waking, and has been proposed as a homeostatic accumulator of the need to sleep (Benington & Heller, 1995; Brown et al., 2012; Landolt, 2008).”

“Convergent pharmacological, genetic, and immunochemical data in vitro suggested that these effects were mediated by an A1 receptor-, cAMP-dependent mechanism (Burke et al., 2015).”

“The ATP levels in wake-active brain regions in rats are elevated during spontaneous sleep and reduced during sleep deprivation (Dworak et al., 2010).”

“…caffeine, theophylline, and paraxanthine act as adenosine receptor antagonists (Biaggioni et al., 1991), and that the trait-like individual differences in the effects of caffeine on sleep have a biological basis in genetic variants coding for adenosine receptors (Retey et al., 2007).”

“Finally, the density of mGluR5 was reported to be reduced in depressed patients (Deschwanden et al., 2011).”

“Both, pharmacological stimulation of A1 and A2A receptors with specific agonists increases slow-wave sleep and EEG slowwave activity (SWA; spectral power in the ~1–4 Hz range) in NREM sleep (Benington et al., 1995; Satoh et al., 1996; Scammell et al., 2001; Schwierin et al., 1996).”

“…and laboratories confirmed that G/A allele carriers exhibited elevated EEG delta power in NREM sleep and enhanced activity in theta/alpha frequencies (~6–12 Hz range) in all behavioural states when compared to G/G allele carriers (Bachmann et al., 2012; Mazzotti et al., 2012; Retey et al., 2005).”

“…acute caffeine intake promotes vigilance, attention and mood by blocking A1 and A2A receptors, particularly in conditions of insufficient sleep and increased sleepiness (Baur et al., 2020; Jarvis, 1993; Landolt et al., 2004; Lieberman et al., 2002; Penetar et al., 1993; Wyatt et al., 2004).”

“Future studies that elucidate whether the beneficial effects of dipyridamole and cannabidiol on RLS symptoms indeed reflect enhanced adenosinergic neurotransmission or another action of these non-selective compounds (e.g., (Friedman & Devinsky, 2015) are warranted.”

“In several regions of the brain, stimulation of adenosine A1 receptors depresses glutamate release, reducing the amplitude of postsynaptic currents (Barrie & Nicholls, 1993; Dolphin & Prestwich, 1985; Oliet & Poulain, 1999).”

“Both, pharmacological stimulation of A1 and A2A receptors with specific agonists increases slow-wave sleep and EEG slowwave activity (SWA; spectral power in the ~1–4 Hz range) in NREM sleep (Benington et al., 1995; Satoh et al., 1996; Scammell et al., 2001; Schwierin et al., 1996).”

“…prospective selection of study participants with predefined ADORA2A genotype is a powerful new paradigm to obtain reliable outcomes in clinical trials testing the responses to coffee, caffeine, and other adenosine receptor antagonists (Baur et al., 2020; Bodenmann et al., 2012; Chen & Cunha, 2020).”

“Next to serotonin, adenosine is an obvious candidate that may influence SCN neuronal activity (Deboer, 2018).”

“Polysomnographic measures of sleep latency were consistently prolonged (Brezinova, 1974; Drapeau et al., 2006; Karacan et al., 1976; Landolt, Dijk, et al., 1995; Okuma et al., 1982; Paterson et al., 2009; Robillard et al., 2015; Youngstedt et al., 2000) and sleep efficiency was reduced (Bonnet &…”

“…to be investigated in detail whether such a role could underlie the caffeine-induced improvement of sleep inertia symptoms (Dornbierer et al., 2021; Van Dongen et al., 2001) and the beneficial effects of short naps on sleepiness that can be potentiated by caffeine intake (so-called…”

“…whether such a role could underlie the caffeine-induced improvement of sleep inertia symptoms (Dornbierer et al., 2021; Van Dongen et al., 2001) and the beneficial effects of short naps on sleepiness that can be potentiated by caffeine intake (so-called “caffeine-naps”) (Reyner & Horne, 1997).”

“…showed reduced SWA and increased activity in the frequency range of sleep spindles (~12– 15 Hz; sigma range) (Drapeau et al., 2006; Landolt, Dijk, et al., 1995; Robillard et al., 2015) even after long periods of wakefulness (Carrier et al., 2009; Landolt et al., 2004; Landolt, Werth, et al., 1995).”

“These hypotheses were tested in a behavioural model of chronic despair in genetically engineered mice (Serchov et al., 2015).”

“…caffeine intake over several days of sleep restriction did not clearly mitigate increased feelings of sleepiness or reduced vigour (Baur et al., 2020; Doty et al., 2017; James & Gregg, 2004) nor continuously reduced sleepiness across consecutive simulated night shifts (Schweitzer et al., 2006).”

“Nevertheless, when the conditional knock-out animals were subjected to prolonged wakefulness, their working memory was impaired and they did not extend their sleep nor showed a normal rebound in EEG SWA in recovery sleep after the sleep deprivation (Bjorness et al., 2009).”

“In humans, higher slow-wave activity (SWA) in the beginning of the night predicts a lower functional magnetic resonance imaging (fMRI) bloodoxygen-level-dependent (BOLD) response in an SCN encompassing region during task performance in the evening (Schmidt et al., 2009).”

“…came from the observations that extracellular adenosine levels in different brain areas in rats and cats tended to increase during wakefulness, decrease during sleep, and to be enhanced after prolonged wakefulness (Huston et al., 1996; Porkka-Heiskanen et al., 1997; Porkka-Heiskanen et al., 2000).”

“First, a combined genome-wide association, replication and neuroimaging study provided convergent evidence that the human gene HOMER1 plays a role in the aetiology of major depression (Rietschel et al., 2010).”

“…alertness, but mainly under conditions of unusually high sleep pressure (Biggs et al., 2007; De Valck et al., 2003; De Valck & Cluydts, 2001; Reyner & Horne, 2000; but see Lohi et al., 2007) and not under normal sleep–wake conditions (Bragg et al., 2017; Kim et al., 2013; Lane, 1994; but…”

“This attenuation can, in the same experiment, be reversed by an A1 receptor antagonist (Elliott et al., 2001; Sigworth & Rea, 2003).”

“…intake was allowed before sleep deprivation (Erblang et al., 2021; Hartley et al., 2013; Walsh et al., 1990) or given over several days (Ataka et al., 2008; Beaumont et al., 2004; James, 1998; Weibel et al., 2020), caffeine did not significantly change subjective sleepiness compared to placebo.”

“On the contrary, brain administration of adenosine and adenosine triphosphate (ATP) in cats and dogs produced behavioural sleep (Feldberg & Sherwood, 1954; Haulica et al., 1973).”

“The release of ATP and glutamate from activated astrocytes has been referred to as “gliotransmission” and is triggered by astroglial Ca2+ signals in proportion to sleep need (Ingiosi et al., 2020).”

“…to sleep homeostasis, mice exhibiting constitutive and central nervous system specific, conditional ablation of A1 receptors showed virtually unaltered sleep–wake behaviour and cognitive functions when studied under baseline sleep–wake conditions (Bjorness et al., 2009; Stenberg et al., 2003).”

“Furthermore, chronic caffeine administration increased the number of adenosine receptors in rats and mice (Boulenger et al., 1983; reviewed by Nehlig et al., 1992).”

“no further rise in adenosine is observed (Chagoya de Sanchez et al., 1993; Murillo-Rodriguez et al., 2004).”

“…also modulate subjective and objective effects of caffeine on sleep quality, sleep architecture and the sleep EEG, it may be that the adenosine system could contribute to the neurobiological mechanism linking disturbed sleep and anxiety/anxiety-related disorders (van Calker et al., 2019).”

“…to elicit feelings of improved alertness, but mainly under conditions of unusually high sleep pressure (Biggs et al., 2007; De Valck et al., 2003; De Valck & Cluydts, 2001; Reyner & Horne, 2000; but see Lohi et al., 2007) and not under normal sleep–wake conditions (Bragg et al., 2017; Kim et…”

“Although there are more potent medications available today, as long as the diagnosis is lacking, many patients with narcolepsy “self-medicate” with high amounts of caffeine to improve their alertness (Thorpy & Dauvilliers, 2015).”

“Additionally, some studies suggest an upregulation in A1 receptors (Boulenger & Marangos, 1989; Ramkumar et al., 1988; Shi et al., 1993; but also see Espinosa et al., 2013; Johansson et al., 1996; Johansson et al., 1997; Nabbi-Schroeter et al., 2018).”

“…Dijk, et al., 1995; Okuma et al., 1982; Paterson et al., 2009; Robillard et al., 2015; Youngstedt et al., 2000) and sleep efficiency was reduced (Bonnet & Arand, 2003; Drapeau et al., 2006; Karacan et al., 1976; Landolt, Dijk, et al., 1995; Okuma et al., 1982; Paterson et al., 2009; Robillard…”

“…that an adaptation to daily caffeine was not observed when sleep was scheduled at the wrong circadian phase, such as after an eastward transmeridian travel (Beaumont et al., 2004), further indicates that caffeine affects sleep not only by acting on the homeostatic facet of sleep– wake regulation.”

“Animal studies suggest that plasma adenosine levels rise during daily intake (Conlay et al., 1997).”

“…may be important (Gass et al., 2010; Pierling et al., 2021), female depressed patients who responded to the serotonin-reuptake inhibitor fluoxetine, also exhibited 30% lower brain ATP levels as quantified with magnetic resonance spectroscopy than treatment nonresponders (Renshaw et al., 2001).”

“Unfortunately, the wakefulness-induced mood improvement is typically short-lived and followed by a relapse into the depressive state upon recovery sleep (Boland et al., 2017).”

“still induce an acute reduction of subjective sleepiness (Adan et al., 2008; Hayashi et al., 2003; Mets et al., 2012; Rogers et al., 2013).”

“Both, pharmacological stimulation of A1 and A2A receptors with specific agonists increases slow-wave sleep and EEG slowwave activity (SWA; spectral power in the ~1–4 Hz range) in NREM sleep (Benington et al., 1995; Satoh et al., 1996; Scammell et al., 2001; Schwierin et al., 1996).”

“…stimulant induced sleep EEG characteristics of insomnia and attenuated the repercussions of prolonged wakefulness on psychomotor vigilance and the SWA rebound in recovery NREM sleep exclusively in C-allele carriers of the ADORA2A polymorphism rs5751876 (Bodenmann et al., 2012; Retey et al., 2007).”

“…occurs after >24 h of abstinence and when sleep pressure is high (i.e., during sleep deprivation or after sleep restriction) (Aggarwal et al., 2011; Beaumont et al., 2001; Dagan & Doljansky, 2006; Hansen et al., 2019; Killgore et al., 2006; Killgore et al., 2012; Killgore & Kamimori, 2020; Kohler…”

“Polysomnographic measures of sleep latency were consistently prolonged (Brezinova, 1974; Drapeau et al., 2006; Karacan et al., 1976; Landolt, Dijk, et al., 1995; Okuma et al., 1982; Paterson et al., 2009; Robillard et al., 2015; Youngstedt et al., 2000) and sleep efficiency was reduced (Bonnet &…”

“still induce an acute reduction of subjective sleepiness (Adan et al., 2008; Hayashi et al., 2003; Mets et al., 2012; Rogers et al., 2013).”

“There is a need for additional studies to clarify whether A1 receptors contribute to the sleep homeostatic feedback signal and A2A receptors provide sleep gating and modulate behavioural sleepiness without controlling sleep homeostasis as suggested on the basis of these insights from genetically modified mice (Lazarus…”

“In several regions of the brain, stimulation of adenosine A1 receptors depresses glutamate release, reducing the amplitude of postsynaptic currents (Barrie & Nicholls, 1993; Dolphin & Prestwich, 1985; Oliet & Poulain, 1999).”

“Also the amplitude of output markers of the circadian clock in humans, like melatonin release or core body temperature do not change under the influence of increased sleep pressure (e.g., Birchler-Pedross et al., 2009; Krauchi et al., 2006), but the sleep pressure induced may not be strong enough.”

“Additionally, some studies suggest an upregulation in A1 receptors (Boulenger & Marangos, 1989; Ramkumar et al., 1988; Shi et al., 1993; but also see Espinosa et al., 2013; Johansson et al., 1996; Johansson et al., 1997; Nabbi-Schroeter et al., 2018).”

“…or after sleep restriction) (Aggarwal et al., 2011; Beaumont et al., 2001; Dagan & Doljansky, 2006; Hansen et al., 2019; Killgore et al., 2006; Killgore et al., 2012; Killgore & Kamimori, 2020; Kohler et al., 2006; Landolt et al., 2004; Lieberman et al., 2002; Paech et al., 2016; Patat et…”

“…of caffeine, insomnia often co-occurs with anxiety/anxiety-related disorders (Jansen et al., 2019) and functional genetic variants in adenosine receptors can predispose people to elevated trait anxiety and anxiety disorders (Geiger et al., 2016; Hohoff et al., 2010; Hohoff et al., 2014).”

“Preclinical studies suggest that brain iron deficiency can cause a hypo-adenosinergic state and a disruption of the fine-tuned balance of adenosine-dopamine-glutamate neurotransmission in the striatum (Ferre et al., 2018).”

“Consistent therewith, self-reported caffeine consumption during “typical days” (Chaudhary et al., 2016) was not associated with daytime sleepiness in a representative sample (Chaudhary et al., 2016).”

“Given the fast kinetics of adenosine formation and breakdown, and the rapidly altered adenosine levels in response to changing vigilance states (Peng et al., 2020), adenosine may play a role in adaptive physiological processes at the transitions between wakefulness and sleep (Hubbard et al., 2020).”

“In addition, Caff seems to slow down the circadian clock (Burke et al., 2015; Oike et al., 2011; Ruby et al., 2018; van Diepen et al., 2014) in both diurnal and nocturnal animals.”

“…in the cerebral cortex correlated with NREM sleep “high delta” (2– 4 Hz) activity in baseline and after sleep deprivation, as well as increased high delta power, elevated sleepiness, and involuntary sleep intrusions during waking after a night without sleep (Hefti et al., 2013; Holst et al., 2017).”

“Importantly, the elevated availability of A1 receptors and mGluR5 after sleep loss returned to baseline levels after recovery sleep (Elmenhorst et al., 2017; Weigend et al., 2019).”

“On the contrary, brain administration of adenosine and adenosine triphosphate (ATP) in cats and dogs produced behavioural sleep (Feldberg & Sherwood, 1954; Haulica et al., 1973).”

“Light information reaches the SCN through the retino-hypothalamic tract (Vansteensel et al., 2008), which releases glutamate at its nerve terminals (Ding et al., 1994; Johnson et al., 1988).”

“Polysomnographic measures of sleep latency were consistently prolonged (Brezinova, 1974; Drapeau et al., 2006; Karacan et al., 1976; Landolt, Dijk, et al., 1995; Okuma et al., 1982; Paterson et al., 2009; Robillard et al., 2015; Youngstedt et al., 2000) and sleep efficiency was reduced (Bonnet &…”

“Also the amplitude of output markers of the circadian clock in humans, like melatonin release or core body temperature do not change under the influence of increased sleep pressure (e.g., Birchler-Pedross et al., 2009; Krauchi et al., 2006), but the sleep pressure induced may not be strong enough.”

“…showed reduced SWA and increased activity in the frequency range of sleep spindles (~12– 15 Hz; sigma range) (Drapeau et al., 2006; Landolt, Dijk, et al., 1995; Robillard et al., 2015) even after long periods of wakefulness (Carrier et al., 2009; Landolt et al., 2004; Landolt, Werth, et al., 1995).”

“On the output side, when analysing rest-activity behaviour (right, here of a night-active animal), SD in most cases, reduces light induced behavioural phase shifts (Burgess, 2010; Challet et al., 2001; Mistlberger et al., 1997; van Diepen et al., 2014).”

“This is supported by the finding that sleep deprivation alone hardly induces phase shifts of the circadian clock in rodents (Challet et al., 2001; Mistlberger et al., 2003; van Diepen et al., 2014).”

“…that the immediate early gene Homer1a and metabotropic glutamate receptors of subtype 5 (mGluR5) may link sleep–wake related changes in A1 receptor signalling to the antidepressant-like effects of sleep deprivation and antidepressant medications in mice (Holz et al., 2019; Serchov et al., 2015).”

“…is <24 h, caffeine has still the potential to elicit feelings of improved alertness, but mainly under conditions of unusually high sleep pressure (Biggs et al., 2007; De Valck et al., 2003; De Valck & Cluydts, 2001; Reyner & Horne, 2000; but see Lohi et al., 2007) and not under normal sleep–wake…”

“The changes in EEG SWA are relatively slow, involving many hours of slow increase during the main waking period and an almost equally slow decreasing process during the main sleep period, independent of the species investigated (Deboer, 2015; Huber et al., 2000).”

“Additionally, some studies suggest an upregulation in A1 receptors (Boulenger & Marangos, 1989; Ramkumar et al., 1988; Shi et al., 1993; but also see Espinosa et al., 2013; Johansson et al., 1996; Johansson et al., 1997; Nabbi-Schroeter et al., 2018).”

“On the output side, when analysing rest-activity behaviour (right, here of a night-active animal), SD in most cases, reduces light induced behavioural phase shifts (Burgess, 2010; Challet et al., 2001; Mistlberger et al., 1997; van Diepen et al., 2014).”

“The glutamate release leads to increased neuronal activity in the SCN (Meijer et al., 1998; van Diepen et al., 2013; van Diepen et al., 2014; van Diepen et al., 2021).”

“…previous waking duration and adenosine on the phase-shifting capacity of the circadian clock, there are no data showing that expression of clock genes in the SCN changes under influence of sleep deprivation, but the data available are limited to one single circadian time point (Curie et al., 2015).”

“…caffeine intake over several days of sleep restriction did not clearly mitigate increased feelings of sleepiness or reduced vigour (Baur et al., 2020; Doty et al., 2017; James & Gregg, 2004) nor continuously reduced sleepiness across consecutive simulated night shifts (Schweitzer et al., 2006).”

“severe dysregulation of sleep–wake homeostasis, including lack of recovery sleep, and impaired behavioural adjustment after sleep deprivation (Holst et al., 2017).”

“In the SCN (middle panel, van Diepen et al., 2014), SD reduces the amplitude of the circadian modulation of SCN neuronal activity (Deboer et al., 2007) and the lightinduced increase in firing rate in SCN neurones (van Diepen et al., 2014).”

“On the output side, when analysing rest-activity behaviour (right, here of a night-active animal), SD in most cases, reduces light induced behavioural phase shifts (Burgess, 2010; Challet et al., 2001; Mistlberger et al., 1997; van Diepen et al., 2014).”

“Restless legs syndrome (RLS) is frequently associated with insufficient iron availability in the brain (Manconi et al., 2021).”

“…analogues dose-dependently increased the durations of nonrapid-eye-movement (NREM) and rapid-eye-movement (REM) sleep, Radulovacki and colleagues were the first to suggest a role for adenosine in the regulation of sleep (Radulovacki, 1985; Radulovacki et al., 1984; Yanik & Radulovacki, 1987).”

“…has still the potential to elicit feelings of improved alertness, but mainly under conditions of unusually high sleep pressure (Biggs et al., 2007; De Valck et al., 2003; De Valck & Cluydts, 2001; Reyner & Horne, 2000; but see Lohi et al., 2007) and not under normal sleep–wake conditions (Bragg…”

“…administration occurs after >24 h of abstinence and when sleep pressure is high (i.e., during sleep deprivation or after sleep restriction) (Aggarwal et al., 2011; Beaumont et al., 2001; Dagan & Doljansky, 2006; Hansen et al., 2019; Killgore et al., 2006; Killgore et al., 2012; Killgore &…”

“…prolonged (Brezinova, 1974; Drapeau et al., 2006; Karacan et al., 1976; Landolt, Dijk, et al., 1995; Okuma et al., 1982; Paterson et al., 2009; Robillard et al., 2015; Youngstedt et al., 2000) and sleep efficiency was reduced (Bonnet & Arand, 2003; Drapeau et al., 2006; Karacan et al., 1976;…”

“…2011; Beaumont et al., 2001; Dagan & Doljansky, 2006; Hansen et al., 2019; Killgore et al., 2006; Killgore et al., 2012; Killgore & Kamimori, 2020; Kohler et al., 2006; Landolt et al., 2004; Lieberman et al., 2002; Paech et al., 2016; Patat et al., 2000; Penetar et al., 1993; Wright, Badia,…”

“Similarly, chronic caffeine intake in adult mice, but not in pubertal rats (Olini et al., 2013), did not elicit clear-cut differences in NREM sleep SWA compared to a control condition (Panagiotou et al., 2019).”

“While sex differences in adenosinergic mechanisms may be important (Gass et al., 2010; Pierling et al., 2021), female depressed patients who responded to the serotonin-reuptake inhibitor fluoxetine, also exhibited 30% lower brain ATP levels as quantified with magnetic resonance spectroscopy than…”

“…used as vasodilator and inhibitor of blood platelet aggregation, proved therapeutic efficacy in RLS by reducing periodic limb movements during sleep, shortening sleep latency, increasing sleep efficiency, and enhancing the duration and proportion of slow-wave sleep (GarciaBorreguero et al., 2021).”

“With respect to adenosine, a small, blood–brainbarrier-permeable positive allosteric modulator of A2A receptors referred to as A2AR PAM-1 was recently shown to promote slowwave sleep in mice (Korkutata et al., 2019).”

“EEG, electroencephalography; (N)REM, (non-) rapid eye movement; SWA, slow-wave activity long an individual must abstain from caffeine to reach a state that is no longer confounded by the prior intake (James, 2014).”

“Sleep in cats under chronic caffeine was shown to normalise in the course of several days after an initial acute increase in waking, but the amount of the deepest stage of sleep remained lower than during baseline (Sinton & Petitjean, 1989).”

“…and laboratories confirmed that G/A allele carriers exhibited elevated EEG delta power in NREM sleep and enhanced activity in theta/alpha frequencies (~6–12 Hz range) in all behavioural states when compared to G/G allele carriers (Bachmann et al., 2012; Mazzotti et al., 2012; Retey et al., 2005).”

“…and when sleep pressure is high (i.e., during sleep deprivation or after sleep restriction) (Aggarwal et al., 2011; Beaumont et al., 2001; Dagan & Doljansky, 2006; Hansen et al., 2019; Killgore et al., 2006; Killgore et al., 2012; Killgore & Kamimori, 2020; Kohler et al., 2006; Landolt…”

“…et al., 2015; Youngstedt et al., 2000) and sleep efficiency was reduced (Bonnet & Arand, 2003; Drapeau et al., 2006; Karacan et al., 1976; Landolt, Dijk, et al., 1995; Okuma et al., 1982; Paterson et al., 2009; Robillard et al., 2015) when caffeine was administered in the evening close to bedtime.”

“…1974; Drapeau et al., 2006; Karacan et al., 1976; Landolt, Dijk, et al., 1995; Okuma et al., 1982; Paterson et al., 2009; Robillard et al., 2015; Youngstedt et al., 2000) and sleep efficiency was reduced (Bonnet & Arand, 2003; Drapeau et al., 2006; Karacan et al., 1976; Landolt, Dijk, et al.,…”

“Moreover, studies in rats revealed that the preferential A2A (but also A1) receptor antagonist, JNJ-40255293, dose-dependently enhanced wakefulness without affecting dopamine and noradrenaline release in prefrontal cortex and striatum (Atack et al., 2014).”

“This attenuation can, in the same experiment, be reversed by an A1 receptor antagonist (Elliott et al., 2001; Sigworth & Rea, 2003).”

“still induce an acute reduction of subjective sleepiness (Adan et al., 2008; Hayashi et al., 2003; Mets et al., 2012; Rogers et al., 2013).”

“Stimulation of A1 receptors inhibits excitatory synaptic transmission by blocking the intracellular build-up of Ca (Chen et al., 1997; Hallworth et al., 2002).”

“…of caffeine, insomnia often co-occurs with anxiety/anxiety-related disorders (Jansen et al., 2019) and functional genetic variants in adenosine receptors can predispose people to elevated trait anxiety and anxiety disorders (Geiger et al., 2016; Hohoff et al., 2010; Hohoff et al., 2014).”

“…latency were consistently prolonged (Brezinova, 1974; Drapeau et al., 2006; Karacan et al., 1976; Landolt, Dijk, et al., 1995; Okuma et al., 1982; Paterson et al., 2009; Robillard et al., 2015; Youngstedt et al., 2000) and sleep efficiency was reduced (Bonnet & Arand, 2003; Drapeau et al., 2006;…”

“, 2007) and not under normal sleep–wake conditions (Bragg et al., 2017; Kim et al., 2013; Lane, 1994; but see Hammami et al., 2010).”

“…(Aggarwal et al., 2011; Beaumont et al., 2001; Dagan & Doljansky, 2006; Hansen et al., 2019; Killgore et al., 2006; Killgore et al., 2012; Killgore & Kamimori, 2020; Kohler et al., 2006; Landolt et al., 2004; Lieberman et al., 2002; Paech et al., 2016; Patat et al., 2000; Penetar et al.,…”

“In rodents, ADA is expressed in sleep–wake regulatory brain regions and exhibits region-specific diurnal variation of enzymatic activity (Chagoya de Sanchez et al., 1993; Okada et al., 2003).”

“However, after sleep deprivation the G/A allele carriers were sleepier than the G/G allele carriers (Bachmann et al., 2012; Reichert et al., 2014).”

“…eight genetic variants of ADORA2A was simultaneously considered, the rebound in EEG SWA in NREM sleep after sleep deprivation differed between two distinct haplotype groups, suggesting that in humans the functional state of A2A receptors modulates the dynamics of sleep homeostasis (Landolt, 2012).”

“An open question in this respect is whether seemingly paradoxical effects of caffeine, such as those anecdotally reported in certain middle-aged individuals (Regestein, 1989), may reflect processes of adaptation.”

“…al., 2001; Dagan & Doljansky, 2006; Hansen et al., 2019; Killgore et al., 2006; Killgore et al., 2012; Killgore & Kamimori, 2020; Kohler et al., 2006; Landolt et al., 2004; Lieberman et al., 2002; Paech et al., 2016; Patat et al., 2000; Penetar et al., 1993; Wright, Badia, Myers, & Plenzler, 1997).”

“…under conditions of unusually high sleep pressure (Biggs et al., 2007; De Valck et al., 2003; De Valck & Cluydts, 2001; Reyner & Horne, 2000; but see Lohi et al., 2007) and not under normal sleep–wake conditions (Bragg et al., 2017; Kim et al., 2013; Lane, 1994; but see Hammami et al., 2010).”

“ATP, adenosine triphosphate; EEG, electroencephalography; (N)REM, (non-) rapid eye movement down under the influence of caffeine (Oike et al., 2011; Ruby et al., 2018; van Diepen et al., 2014).”

“…larger studies confirmed this association (Tiechelmann et al., in preparation) and suggested that C-allele homozygotes exhibit increased time awake (as estimated from wrist actigraphy) and more frequent sleep complaints when compared to Tallele carriers (Erblang et al., 2021; Nova et al., 2012).”

“There is evidence in the animal domain that caffeine's main metabolite paraxanthine has similar affinity as caffeine (Snyder et al., 1981) to both receptors (Chou & Vickroy, 2003) and disturbs NREM sleep (Okuro et al., 2010).”

“…of caffeine, insomnia often co-occurs with anxiety/anxiety-related disorders (Jansen et al., 2019) and functional genetic variants in adenosine receptors can predispose people to elevated trait anxiety and anxiety disorders (Geiger et al., 2016; Hohoff et al., 2010; Hohoff et al., 2014).”

“Conversely, caffeine, the most widely consumed psychostimulant, and a nonselective adenosine receptor antagonist, produces a strong arousal effect (22-24).”

“Furthermore, the changes in adenosine in response to increased vigilance (upon waking and in response to stimuli) occur on the order of minutes, not hours [210].”

“exerts its alerting effects by antagonizing adenosine 2A receptors (Huang et al., 2005; Reichert et al., 2022).”

“N3 sleep is linked to homeostatic sleep regulation, with a tendency for greater N3 sleep when sleep pressure is high [53, 54].”

“Also, immense literature has been reviewed on the behavioral effects of caffeine (Frigerio et al., 2021; Gardiner et al., 2023; James, 2014; Reichert et al., 2022), pharmaceutical applications of caffeine in neurodegenerative diseases (Kolahdouzan & Hamadeh, 2017; Lu'o'ng & Nguyen, 2015; Pohanka,…”

“Adenosine is a neuromodulatory molecule that inhibits the activity of neurons responsible for wakefulness; therefore, it is considered a sleep-inducing substance (Porkka-Heiskanen, 1999; Reichert et al., 2022).”

“Some studies suggest that regular caffeine users may develop a tolerance to caffeine-induced sleep-disrupting effects [132, 133] while others indicate that habitual consumption does not eliminate these disturbances.”

“The CAF probably affects the circadian rhythm of the body indirectly by making it more sensitive to phase shifts caused by light [71].”

“The increased cytosolic cAMP/Ca2+ activates the protein kinase A (PKA) and Ca2+/calmodulin-dependent protein kinase Ⅱ (CaMKⅡ), thereby leading to the phospho-dependent activation of cAMP response element binding protein (CREB), which in concert with its coactivators to activate the cAMP response element (CRE) (Narishi…”

“…related to TRPV1 in terms of distribution in skin-innervating nociceptive neurons and activation by nox-ious stimuli [73] and is responsive to an incredibly wide spectrum of activating stimuli from exogenous and endogenous irritants, low pH, osmotic pressure, and extremes in temperature [47,73–78].”

“Complementary insights can be harnessed by assessing brain signal complexity using entropy metrics.”

“Caffeine enhances alertness, activates the cortex, lessens fatigue, and impedes adenosine receptors, inducing wakefulness and diminishing cortical slow-wave activity [39,40].”

“Interestingly, together with physical exercise, smoking and caffeine/coffee intake (despite the general idea that acute and chronic intake of caffeine is associated with less sleep or more disturbed sleep [45,46]) are considered neuroprotectant factors against the development of PD [47].”

“Effects of both caffeine and sleep loss are mediated in part by the adenosine system 19,20 .”

“The theory of sleep homeostasis 56 suggests that the pressure or need for sleep increases with time spent awake and decreases during sleep.”

“These outcomes are supported by the wakefulness induced by the pharmacological antagonism of adenosine receptors by caffeine or by genetic modification in adenosine receptor knock-out genotypes (Reichert et al. 2022).”

“Even the best-characterized factors hypothesized to mediate sleep pressure do not fully account for the daily rise and fall of sleep pressure during regular sleep/ wake cycles [4].”

“…is currently unknown whether caffeine concentrations below the limit of detection, an attenuated build-up of sleep pressure by blockading adenosine receptors during wakefulness, or active metabolites such as paraxanthine under-pin the effect of caffeine on sleep intensity (Reichert et al., 2022).”

“…blockade of adenosine receptors (A1/A2A), while high doses of caffeine, by elevating cortisol levels and activating the stress system, may disrupt sleep architecture (e.g., prolonging sleep latency, reducing slow-wave sleep, shortening total sleep time, and worsening perceived sleep quality) (30).”

“This function occurs following acute caffeine intake, while the chronic effects of caffeine have not yet been fully elucidated in the scientific literature [28,29].”

“Adenosine is an inhibitory neuromodulator involved in sleep-wake regulation — as extra-cellular adenosine concentrations increase, so does sleepiness (Basheer et al., 2004; Huang et al., 2011; Porkka-Heiskanen et al., 1997; Reichert et al., 2022).”

“Caffeine is a psychoactive stimulant that reduces tired-ness by antagonizing adenosine receptors in the brain [9, 10].”

“However, our findings are consistent with existing biological evidence: firstly, caffeine disrupts sleep structure (e.g., prolonging sleep latency, reducing slow-wave sleep, decreasing total sleep time) by increasing cortisol and activating the stress system, leading to decreased sleep quality (34).”

“Caffeine modulates the circadian response to light [41].”

“[8] Furthermore, studies have shown that caffeine dependence may develop among individuals who consume caffeine because of a subliminal need to counteract sleep inertia.”

“During the day, adenosine triphosphate (ATP) is degraded to adenosine, which then accumulates in the extracellular space [221].”

“The muscular and plasma concentration of adenosine increases during physical activity [30] resulting in an activation of central adenosine receptors that promote reduced neuronal excitability and synaptic transmission due to the inhibition of neurotransmitters in the brain [31].”

“Adenosine can be considered a central excitatory and inhibitory neurotransmitter and an important sleep-regulating substance [40,41].”

“It was suggested that caffeine reduces subjective sleepiness until the body becomes accustomed [Reichert et al ., 2022].”

“Hippocampal rhythms are related to the stage of sleep, and adenosine levels in the brain are proposed to act as a regulator of sleep (Reichert et al., 2022).”

“As an antagonist to the adenosine receptors, it inhibits the sleep promoting effects of adenosine and therefore results in wakefulness and alertness [192,193].”

“This adaptation is largely mediated by the compensatory upregulation of central adenosine receptors in response to chronic antag-onism by caffeine (Reichert et al., 2022; Lopes et al., 2023).”

“Intra-and extracellular adenosine concentrations are controlled by complex regulatory processes which depend on the metabolic state of both neurons and astrocytes [7].”

“It thus stands to reason that caffeine increases alertness and wakefulness by competitively inhibiting the binding of adenosine to its receptors (Reichert et al. 2022).”

“The activation of A1 receptors has been linked to antidepressant-like e ff ects and an improved sleep quality [31,32], suggesting that LVE may contribute to sleep regulation through adenosine A1 receptor activation.”

“Neuronal production of adenosine is also higher during waking hours than during sleep, allowing the promotion of wakefulness as morning approaches [12,14].”

“For the MR-Egger method, we removed the variables related to adenosine receptors (Reichert et al., 2022), as these influence sleep most directly.”