Understanding Sleep
Circadian Rhythms
Sleep, in humans and mammals, is a complex process regulated by two distinct processes – a homeostatic process determined by the amount of time spent asleep/awake and an internal clock-like process of circadian rhythmicity that is largely independent of prior time spent asleep or awake.
A circadian rhythm is an endogenously generated time-keeping cycle within the brain. Each cycle is approximately 24 hours in duration, corresponding with the 24-hour light–dark cycle produced by the 24-hour rotation of the earth.1 Circadian rhythms are regulated by a biological (or circadian) clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus (see Figure 1), also known as the circadian pacemaker.
In mammals, the circadian clock governs vital homeostatic functions such as sleep, wakefulness, temperature, feeding, neuroendocrine, and autonomic rhythms.2 On a daily basis, the circadian clock adjusts its period to compensate for differences between the intrinsic period and the 24-hour period of the environmental day. This process of ‘entraining’ the body’s internal clock to the 24-hour day is synchronised by an environmental source, which, for most mammals, is the daily light–dark cycle.2,3 The environmental cues essential for the entrainment of circadian rhythms are known as ‘zeitgebers’. Light is the most important environmental zeitgeber that has an effect on circadian rhythms. Other zeitgebers include feeding, temperature, and activity. In the absence of zeitgebers, desynchronisation occurs, resulting in ‘freerunning’ circadian rhythms with an intrinsic period of approximately 24 hours (24.2–25 hours).1,4 Misalignment of the circadian clock with these environmental cues may result in sleep disorders, in which the endogenous clock runs faster or slower than the norm.2
Read more about the interaction of circadian rhythms with the sleep–wake cycle in the regulation of sleep.
References:
1. Andrews A, Benbadis S. Chronobiology and circadian rhythms. In: Carney PR, Berry RB, Geyer JD, eds. Clinical Sleep Disorders. Philadelphia: Lippincott Williams & Wilkins, 2005: 85–94.
2. Zisapel N. Circadian rhythm sleep disorders. Pathophysiology and potential approaches to management. CNS Drugs 2001; 15 (4): 311–328.
3. Miller JD, Morin LP, Schwartz WJ, Moore RY. New insights into the mammalian circadian clock. Sleep 1996; 19 (8): 641–667.
4. Elmore SK, Betrus PA, Burr R. Light, social zeitgebers, and the sleep–wake cycle in the entrainment of human circadian rhythms. Res Nursing Health 1994; 17: 471–478.