Circadian Rhythms

Sleep–wake cycle

The most widely recognised circadian rhythm in humans is the sleep– wake cycle, but other examples include body temperature, blood pressure, production of hormones, and digestive secretions. The human sleep–wake cycle is not simply driven by the circadian pacemaker located in the SCN, but involves dual regulation by circadian rhythmicity and a sleep–wake oscillatory process – sleep homeostasis (i.e., the balance between sleep and waking). The principle of sleep homeostasis is simply that, in response to a lack of sleep there is an increased need for sleep and that the duration of any subsequent sleep period is longer than that observed in normal sleep. The sleep homeostat, presumably located outside the SCN, is strongly determined by the sleep–wake cycle. Since we close our eyes to sleep and they are open when we are awake, the sleep–wake cycle is also a major determinant of the light input into the circadian clock. The determination of light input by the sleep–wake cycle is mediated by circadian photoreception (see Figure 1).¹

In humans, the SCN is thought to generate a ‘wake’ or ‘arousal’ signal that increases in strength throughout the biological day (i.e., during the habitual wake episode), peaking in the evening hours at ~22:00 hours. The strength of this signal declines during the biological night (i.e., during the habitual sleep period,^2,3 to reach a minimum at ~06:00 hours, which coincides with the body temperature nadir (lowest daily body temperature – an established marker of the output of the circadian pacemaker).² In the absence of the circadian arousal signal generated by the SCN, sleep–wake consolidation is lost, and the usual monophasic sleep–wake cycle is replaced by a polyphasic sleep–wake cycle, presumably dictated primarily by sleep homeostasis. Therefore, the primary role of the circadian pacemaker in the sleepregulation cycle is to maintain the timing of the cycle and the consolidation of the sleep–wake behaviour by opposing the increase in the need for sleep (homeostasis) associated with sustained wakefulness.^1,2

References:
1. Dijk D-J, Lockley SW. Invited Review: Integration of human sleep–wake regulation and circadian rhythmicity. J Appl Physiol 2002; 92: 852–862.
2. Dijk D-J, Czeisler CA. Paradoxical timing of the circadian rhythm of sleep propensity serves to consolidate sleep and wakefulness in humans. Neurosci Lett 1994; 166: 63–68.
3. Edgar DM, Dement WC, Fuller CA. Effect of SCN lesions on sleep in squirrel monkeys: evidence for opponent processes in sleep–wake regulation. J Neurosci 1993; 13: 1065–1079.