We are all affected by a 24-hour cycle known as the circadian rhythm, also known as the "human clock." Our bodies pass through this natural cycle and help us to differentiate day and night. Hormones and temperature changes differentiate sleep cycles from awake cycles.
CIRCADIAN RHYTHM IN BABIES
When in the womb, baby receives melatonin, the hormone that governs sleep cycles, from mom. At birth, they have very little left. Small tummies and lack of melatonin mean that it takes 2-3 months before baby starts to fall into a more normal sleep pattern. Infants go through 3- to 4-hour cycles of sleep and wakefulness through the night. The sleep-wake schedule will consolidate once the circadian rhythm of the infant matures. Once your baby is past 3 months, you can help encourage the night / day differentiation by having a regular, if not set, feeding pattern and having a good bedtime routine.
Of course, getting your baby to fall asleep - and stay asleep - is as much about your baby's temperament as it is about these natural rhythms. Not all babies will be able to naturally extend their own sleep, or go to sleep on their own, without some assistance from mom & dad. Sleep training can help you tailor a sleep routine to your baby & his / her temperament.
CIRCADIAN RHYTHM IN PARENTS
Parents, too, have a major adjustment during the initial months because of the circadian rhythm. Our bodies are regulated to sleep mode during the night so waking up when our hormone and body temperature levels are low can be quite difficult. Sleep deprivation can become quite a problem for new parents, contributing to feelings of depression and anxiety and marital problems.
Resources referenced: Baby Center, ezine, wikipedia