National Sleep Foundation

Chapter 1: Normal Sleep

Sleep Regulation

Sleep Regulation

Circadian and homeostatic influences help regulate sleep and wakefulness over a 24-hour period.

The homeostatic sleep drive (i.e., sleep need) is near zero when a person wakes up spontaneously in the morning, and grows continually the longer he or she is awake. The longer we are awake, the drowsier we get. 

But that is not all. What keeps us at the (relatively) same high level of alertness and wakefulness all throughout our natural waking hours is a circadian wakefulness-promoting signal that is generated by a group of cells on either side of our brain, called the “suprachiasmatic nucleus” (SCN). The SCN is located in the hypothalamus immediately above the optic chiasm, and receives input from the retina. It not only keeps us fully awake as the day progresses, but also is genetically controlled to provide circadian rhythmicity to nearly every physiological system.1, 2, 3, 4

As the time for sleep approaches, the circadian waking signal drops, allowing the homeostatic sleep drive to become dominant (see Figure 1.5).5, 6  This circadian and homeostatic interaction offers insight why, after a sleepless night, people seem to feel less tired at noon than they do in the morning.

Rapid eye movement (REM) sleep and the time of spontaneous awakening from sleep are more circadian-driven processes, while slow-wave sleep (SWS) is closely related to the homeostatic sleep drive (i.e., the more sleep deprivation a person experiences, the more SWS happens the next night). Clearly, external influences can affect this relationship; for example, a person may be able to stay awake all night to care for a sick person, when they would normally be asleep instead.7


Figure 1.5: Sleep-wake cycle: the role of circadian regulation8


Circadian Rhythm

The circadian rhythm typically emerges two to six months after birth and is entrained by light to Earth’s daily rotation (i.e., adjusted to a 24-hour cycle).9  The circadian rhythm of people who are “early birds” may be less than 24 hours (i.e., when the external clock is 9:00 PM, an early bird’s internal body time may already be later, like 12:00 AM), while the rhythm of “night owls” rhythm may run longer (i.e., when the external clock is 9:00 PM, a night owl’s internal body time may be earlier, like 5:00 PM).

This rhythm seems to vary over the life cycle: adolescents and young adults are typically night owls, while older people are typically early birds.

Without time cues, individuals maintain a free-running biological rhythm that is usually just slightly longer than 24 hours, although this can vary.10 

Figure 1.6 (below) shows a 55-day recording of sleep-wake cycles in a normal volunteer. For the first 20 days, the volunteer had a watch and was instructed to go to bed at 12:00 AM and wake up at 8:00 AM. The following 35 days of the study occurred in an environment without any time cues, in which the volunteer turned the light on or off as desired. Over time, the volunteer developed a free-running cycle of 25.3 hours.11  (Blind people, who lack light clues, have a free-running 25-hour cycle, which causes them unique sleep/wake problems.)


Figure 1.6: Human sleep-wake cycle in a temporal isolation environment12






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