Sleep is a period of inactivity with raised thresholds to arousal by external stimuli. However sleep is an active process. While some brain areas are inhibited others remain active. This activity is sometimes called beta activity. In Stage 1, the initial period of sleep, alpha waves gradually decrease in amplitude and become more irregular in frequency.
Species differ in the type of sleep they display.
Cycles earlier in the night are shorter and REM is more prominent in the second half of the night. These include the brain stem reticular formation, raphe nuclei, basal forebrain area, and locus coeruleus. There are large individual differences in the amount of sleep needed but most healthy young adults require 7 to 8 hours. Sustained sleep deprivation in animals can be fatal. In humans the behavioural effects of prolonged sleep deprivation may depend on personality, age and motivation.