Physiology of Sleep and Wakefulness Most adults sleep 7–8 h per night, although the timing, duration, and internal structure of sleep vary among healthy individuals and as a function of age. At the extremes, infants and the elderly have frequent interruptions of sleep. In the United States, adults of intermediate age tend to have one consolidated sleep episode per day, although in some cultures sleep may be divided into a midafternoon nap and a shortened night sleep. Two principal systems govern the sleep-wake cycle: one actively generates sleep and sleep-related processes and another times sleep within the 24-h day. .
