New borns sleep 16-18 hours a day in the first 3 months. Babies sleep in a polyphasic (many times in 24 hours) sleep order for equial number of hours during night and day and they wake up and fall asleep again in many times. Fortunately, this excruciatingly difficult sleep order for parents does not last very long.
Starting from 3rd month, babies start to sleep less time. They begin to wake up less frequently from their night sleeps. When babies are 6 months old, their need for sleep reduce to 12 hours and they can stay awake longer during the daytime.
In preschool period, it becomes to biphasic (2 times in 24 hours) sleep of adulthood; and during the daytime, child starts to sleep only once in the afternoon. This sleep order later becomes a non-physiological, monophasic - ie once in 24 hours - night sleep because of going to school and later, because of the difficulties of business life.
Another characteristic of children's sleep is being consisted of mainly REM sleep. While half of sleep time of newborns is REM phase; this phase gradually decreases and when they are at school age, the level reaches to 20-25% of adult’s level.