To feel rested and refreshed upon awaking, most adults require 7–8 hours of sleep, although this number varies among individuals (Figure 28.1A). As a result, a substantial fraction of our lives is spent in this mysterious state. For infants, the requirement is much higher (about 16 hours a day), and teenagers need on average about 9 hours of sleep. As people age, they tend to sleep more lightly and for shorter times, although often needing about the same amount of sleep as in early adulthood (Figure 28.1B). Getting too little sleep creates a “sleep debt” that must be repaid in the following days. In the meantime, judgment, reaction time, and other functions are impaired. Drivers who fall asleep at the wheel are estimated to cause some 56,000 traffic accidents annually and 1,500 highway deaths.

Figure 28.1

The duration of sleep. (A) The duration of sleep each night in adults is normally distributed with a mean of 7.5 hours with a standard deviation of about 1.25 hours. Thus, about two-thirds of the population sleeps between 6.25 and 8.75 hours each night. (more...)

Sleep (or at least a physiological period of quiescence) is a highly conserved behavior that occurs in animals ranging from fruit flies to humans (Box A). This prevalence not withstanding, why we sleep is not well understood. Since animals are particularly vulnerable while sleeping, there must be advantages that outweigh this considerable disadvantage. Shakespeare characterized sleep as “nature's soft nurse,” noting the restorative nature of sleep. From a perspective of energy conservation, one function of sleep is to replenish brain glycogen levels, which fall during the waking hours. In keeping with this idea, humans and many other animals sleep at night. Since it is generally colder at night, more energy would have to be expended to keep warm were we nocturnally active. Furthermore, body temperature has a 24-hour cycle, reaching a minimum at night and thus reducing heat loss. As might be expected, human metabolism measured by oxygen consumption decreases during sleep.

Box A

Styles of Sleep in Different Species.

Whatever the reasons for sleeping, in mammals sleep is evidently necessary for survival. For instance, rats completely deprived of sleep die in a few weeks (Figure 28.2). Sleep-deprived rats lose weight despite increasing food intake, and progressively fail to regulate body temperature. They also develop infections, suggesting an impairment of the immune system.

Figure 28.2

The consequences of total sleep deprivation in rats. (A) In this apparatus, an experimental rat is kept awake because the onset of sleep (detected electroencephalographically) triggers movement of the cage floor. The control rat can thus sleep intermittently, (more...)

In humans, lack of sleep leads to impaired memory and reduced cognitive abilities, and, if the deprivation persists, mood swings and even hallucinations. The longest documented period of voluntary sleeplessness is 264 hours (approximately 11 days), a record achieved without any pharmacological stimulation. The young man involved recovered after a few days, during which he slept only somewhat more than normal, and seemed none the worse for wear.