How much time people sleep at night varies greatly with their age. Broadly speaking, from birth to death, the amount of sleep we get decreases steadily.
Newborns sleep an average of 16 hours per day, but even at this age, some babies sleep a lot more (20 hours) while others sleep a lot less (12). Newborns’ sleep is not affected by the alternation of day and night. Instead, it is broken up into periods of three or four hours, and the main thing that wakes newborns up is the need to nurse. Infants spend about half of their sleeping lives in REM sleep–double the proportion for adults.
This large amount of REM sleep in infants is believed to assist the development of their central nervous systems. Scientists know that neural activity helps developing synapses to find their targets. This neural activity is very intense during REM sleep, so the frequent episodes of REM sleep in babies may facilitate the activation of neural pathways and the establishment of the appropriate synaptic connections between them.
Babies develop a circadian rhythm when they are somewhere between one month and six months old. At that point, they begin to sleep through the night (much to their parents’ relief) and their sleep becomes mainly nocturnal (for example, 10 hours through the night, and two or three naps for a total of 6 hours in the daytime).
The average amount of sleep that children get per day declines steadily as they grow older–from 15 hours when they are six months old, to 14 when they are two years old, and then to 12 (10 hours at night and a two-hour nap in the daytime) when they are about three or four years old. Their proportion of REM sleep declines rapidly until age four, when it stabilizes at the same level as a young adult’s: about 20 to 25 percent of the total time spent asleep.
Children around 10 years old sleep just about 10 hours per night. Teenagers still need more sleep than adults–around 8 ½ to 9 hours. Teens’ biological clocks also make them stay awake later into the night and stay asleep later into the morning. That’s why classes that start early in the morning aren’t the greatest idea for teenage students.
In older people, sleep is often lighter and more fragmented. Older people also tend to get up earlier in the morning. They don’t sleep as long at night, but they continue to need about the same total amount of sleep as young adults, and therefore need to take naps in the daytime to make up for their shorter sleep at night.
In older people, REM sleep decreases to about 15 percent of total sleeping time. The deepest form of sleep (Stage Four non-REM sleep) also diminishes gradually with age, so that older people’s sleep is more susceptible to disturbances of all kinds. Given the importance of non-REM sleep for the immune system, it may well be that this reduction in non-REM sleep also makes older people more vulnerable to illness.