A neural structure that is central to wakefulness is the reticular activating system in the brainstem — notably the raphe nuclei (associated with serotonin), the locus coeruleus (projecting norepinephrine) and the peribrachial nuclei (projecting acetylcholine) — which activates the higher brain centers. Lesions of the raphe nuclei result in insomnia. All three neuromodulators are actively projecting in the waking state, and all three are less active during non-REM sleep (SWS). Acetylcholine from the brainstem activates both the histamine-producing tubermammillary nucleus in the hypothalamus and the basal nucleus of the forebrain, which itself sends acetylcholine to the cerebral corex as an arousal-promoting neuromodulator. The basal nucleus is also activated by norepinephrine from the locus coeruleus. Both serotonin & noradrenaline levels drop to near zero in REM sleep, but acetylcholine levels are as high or higher in REM than in waking [SCIENCE; Maguet,P; 294:1048-1051 (2001)]. Acetylcholine enhances cortical plasticity. Serotonin, on the other hand, can inhibit REM sleep [SLEEP; Datta,S; 26(5):513-520 (2003)]. Cortisol also inhibits REM sleep, and the decline of REM sleep with senescence is primarily due to increased plasma cortisol [THE JOURNAL OF CLINICAL ENDOCRINOLOGY & METABOLISM; Vgontias,AN; 88(5):2087-2095 (2003)].
Adenosine acts like a neurotransmitter in the brain, but is not released from synapses. Instead, adenosine is released by both neurons and glial cells as a result of the breakdown of the energy-supplying ATP (Adenosine TriPhosphate) molecules. Thus, a rise in brain adenosine is associated with a depletion of energy in the brain cells. Appropriately, the action of adenosine on brain adenosine receptors is to induce sleepiness (so that resting can replenish ATP) [THE JOURNAL OF NEUROSCIENCE; Thakkar,MM; 23(10):4278-4287 (2003)]. Caffeine blocks the action of adenosine receptors. Individuals who are more sensitive to the effects of caffeine are also more sensitive to the effects of sleep deprivation [THE JOURNAL OF NEUROSCIENCE; Refey,JV; 26(41):10472-10479 (2006)].
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