Circadian rhythms refer to patterning of biological rhythms, including
sleep and wake periods, that occur across a 24-hour cycle [33]. Circadian
rhythms work with sleep homeostasis to maintain discrete periods of
alertness and sleepiness. The two-process model proposed by Borbely
describes how the homeostatic (S process) and the circadian process (C
process) work in conjunction (Figure 2.5) [34]. Sleep homeostasis refers
to the pressure to sleep that accumulates as the duration of wakefulness
increases. For example, the homeostatic drive to sleep decreases as the
sleep need is met.
Circadian regulation of sleep is not influenced by the amount of preceding
sleep, but rather is controlled by an endogenous biological clock
located in the suprachiasmatic nucleus (SCN) of the hypothalamus (Figure
2.6) [35,36]. These rhythms typically run longer than 24 hours but they
become synchronized to the 24-hour clock via environmental cues (eg,
the light–dark cycle). Sunlight is the strongest ‘zeitgeber,’ or influence,
on the timing of sleep and wake. For example, sunlight impacts sleep
and wakefulness through its impact on the secretion of melatonin, a
hormone which promotes sleep [37]. This is due to sunlight transmitted
through the retina along the optic nerve to the SCN in the hypothalamus,
which regulates melatonin, body temperature, and other functions that
contribute to sleep. Specifically, the pineal gland of the SCN produces
melatonin at night