Babies need good sleep as much as they need good nutrition. While asleep, babies rest, grow and develop. Under-slept babies tend to cry more, to be difficult to comfort and difficult to play with, and therefore do not learn well.
Sleep is not a single state, but at least two: Rapid Eye Movement (REM) and non-REM sleep. During the light REM sleep the baby is dreaming and can twitch, and breathe irregularly. Eye movements can be seen under the eyelids. Non-REM sleep is deeper, and the baby will breathe deeply, be very still, and may occasionally make sucking motions. These alternate in a cycle between 30 and 45 minutes long, but these can be shorter in babies under the age of six months. Between two cycles babies rouse briefly and can appear to be awake.
The baby needs help to learn to sleep for at least two cycles during the day sleeps, to increase the length of night time sleeps, and to have adequate total sleep in twenty-four hours.