During pregnancy the fetus’s body is synchronized to the mother. Body temperature and pulse go up and down with the mother’s, as do melatonin levels. Even though there are no light signals to the fetus’s optical system, the melatonin cycle syncs with light experienced by the mother. Fetuses have sleep stages too. Not the same as the sleep cycles for adults and children, but scientists have been able to identify periods of varying activity in the fetal brain corresponding, more or less, to NREM and REM sleep. Like newborns, fetuses spend a significant percentage of their sleep in REM. It is believed that the function of sleep in fetuses is the same as it is in adults.
However, like a newborn baby, the third-trimester fetus does not show a regular pattern of sleep anything like that of adults. Pregnant women often find this frustrating and wish the unborn baby would sleep when the mother is asleep. Fetuses also yawn.
The sleep function is largely unchanged by birth. Researchers have found significant continuity from the final week of pregnancy to the neonatal period.
Infants sleep a lot, more than half the day. In the first few months the infant typically sleeps 16 to 18 hours per day. Half of this is REM sleep and half is deep sleep. It’s not just human babies who experience huge amounts of REM sleep. Other altricial species such as dogs and cats (altiricial means the newborns require major parental attention, being unable to fend for themselves and often to move much or see right after birth.)
The circadian clock is unformed in infants and one of the things that the infant body learns to do as it grows is to entrain to the sunlight. Indeed, infants appear to follow random sleep patterns, paying little heed to the clock of the adult world of time. Newborns spend about 8 hours per day awake, 8 hours in slow-wave sleep, and 8 hours in REM sleep. The newborn baby needs feeding every 4 hours and will awake if hungry. The sleep regulation is dominated by the homeostatic process. The circadian regulation is minimal although as the baby grown its patterns become more entrained with the environment.
Parents of newborns suffer from sleep deprivation because they have to get up with the baby so often. Babies are different in their sleep patterns, and sometimes a baby is said to be a rare one who sleeps through the night. Of course, like adults, these babies do wake briefly during the night, but they are quiet and go back to sleep without fussing, so they seem to someone elsewhere in the house to be sleeping through the night. For both infants and toddlers, the ability to sleep through the night is a genetic thing; the older the child gets the more the environment and family life plays a part in sleep patterns.