Sleep

Humans are generally diurnal. The average sleep requirement is between seven and nine continuous hours a day for an adult and nine to ten hours for a child; elderly people usually sleep for six to seven hours. Experiencing less sleep than this is common in modern societies; this sleep deprivation can have negative effects. A sustained restriction of adult sleep to four hours per day has been shown to correlate with changes in physiology and mental state, including fatigue, aggression, and bodily discomfort.