Newborn babies sleep for periods throughout the day and night. By the age of two or three months, many babies have learned to sleep through the night, though they nap for periods during the day. By the age of four, most children have given up daytime naps. Most adults need slightly less sleep as they grow older. A person who slept 8 hours a night at 30 years of age may need only 7 hours of sleep at age 60.
Normal sleep consists of two types.
The first is known as slow-wave sleep, because during it there is reduced electrical activity in the brain. During slow-wave sleep, there is a decrease in the basal metabolic rate, blood pressure, and respiratory rate so that the person is relaxed. The second type of sleep is known as paradoxical or REM sleep, because of the rapid eye movements that take place behind the closed eyelids. Also during this phase, dreaming takes place and the heartbeat and respiration become irregular, but there is no limb movement. An electroencephalogram shows electrical brain activity similar to that which occurs when a person is awake.