I have many parents coming to me and asking me questions about a baby's sleeping behaviour. Now, a normal baby who is less than three months of age sleeps for almost 14-16 hours a day, though some may sleep less. During the time that the baby is awake, and this is most often at night, the baby wants to be constantly fed. Even immediately after she has completed her feed, she will continue to be awake and will often be irritable/playful in turns. To a mother who feels harassed that her baby is keeping her awake at nights, I have only to say this: the baby is programmed to stay up at nights because her mother gets the maximum breast milk output only at night. This is true not only for humans but for most mammals.
As the baby grows older, the sleeping hours gradually start falling. By the age of six months, the sleeping hours have declined to 12-14, by one year, to about 10-12 and by five years, to about 9-10 hours per day.
The first few months of night-time awakenings are gradually replaced by the baby learning to sleep during the nights and staying awake in the day. However, this transition period may get prolonged in some babies, and I have seen babies even as old as one and a quarter years of age who haven't learned to sleep well at night. One of the factors responsible for such a delay is the institution of bottle-feeding - esp. the bottle that is offered to the baby when she is going to sleep. The baby associates the bottle in her mouth with the sleep to such an extent, that if the bottle is not provided, she just won't fall asleep.
My advice to all mothers is not to encourage the night-time bottle at all - in fact, to never use a feeding bottle at all, if possible.
During the first few hours of a full night's sleep, both children and adults pass into deeper and deeper sleep till they are in deep sleep within an hour to ninety minutes. After this first spell of fitful sleep, which ends around 2 - 2 1/2 hours, the baby/child/person gradually emerges to a near conscious state and passes into her first dream-stage sleep, the so-called "REM" (Rapid Eye Movements) sleep. During this sleep, the brain is active (alpha waves), as are the muscles (tight), the mouth (chewing movements with or without a tightly held lower jaw), the eyes (moving side to side rapidly with blinking of the lids ... hence the name REM) and almost the entire body ... internal organs and all.
After the REM stage, which lasts for 15-20 minutes, the baby/person passes into the second bout of ever-deepening sleep. This, however, is not as long as the first bout, and may get over in less than an hour. A repeat REM follows. This alternation between deep sleep and REM sleep continues throughout the night. However, the REM phases come sooner and sooner, so that by the early hours of the morning, REM stages are repeated with just half an hour gaps in between. Finally, after the last spell of REM, the baby/child/person wakes up, fresh and eager for the day.
Understanding the above pattern is extremely useful for both, the person who sleeps and the others who look after that person. If, for example, you were to disturb a person who is still in the REM stage, the baby/child/person will wake up irritable, sleep-deprived, tense and uninclined to arise and shine.
To understand sleep is to understand rejuvenation of the human body. Every night, the tired individual sleeps, rejuvenates, and wakes up in the morning, relaxed and refreshed, ready to face the hurdles of life anew.