Infant sleep
Infant sleep is an act of sleeping by an infant or a newborn. It differs significantly from sleep during adulthood. Unlike in adults, sleep early in infancy initially does not follow a circadian rhythm. Infant sleep also appears to have two main modes - active, associated with movement, and quiet, associated with stillness - exhibiting distinct neurological firing patterns. Sleep duration is also shorter. As the infant ages, sleep begins to follow a Circadian rhythm and sleep duration increases. Infants nap frequently. Infants are also particularly vulnerable during sleep; they are prone to suffocation and SIDS. As a result, "safe" sleep techniques have been the subject of several public health campaigns. Infant sleep practices vary widely between cultures and over history; historically infants would sleep on the ground with their parents. In many modern cultures, infants sleep in a variety of types of infant beds or share a bed with parents. Infant sleep disturbance is common, and even normal infant sleep patterns can cause considerable disruption to parents' sleep. As a result, there are a wide variety of interventions and products intended to improve infant sleep; however, their efficacy is variable. Infant sleep training is one common intervention for poor infant sleep. Poor infant sleep is linked to maternal depression. Normal infant sleep 
In the first week of life, infants will sleep during both the day and night and will wake to feed. Sleep cycle duration is usually short, from 2–4 hours. Over the first two weeks, infants average 16–18 hours of sleep daily. Circadian rhythm has not yet been established and the infants sleeps during the night and day equally. In the first month of life, 95% of infants will wake during the night. At around 2 months, a day-night pattern begins to gradually develop. At around 3 months, sleep cycle may increase to 3–6 hours, and the majority of infants will still wake in the night to feed. By 4 months, the average infant sleeps 14 hours a day (including naps), but this amount can vary considerably. By 8 months, most infants continue to wake during the night, though a majority are able to fall back asleep without parental involvement. At 9 months, only a third of infants sleep through the night without waking. Daytime sleeping (naps) generally doesn't cease until 3 to 5 years of age. Infant sleep in the first year can be categorised into active sleep (AS) and quiet sleep (QS). Active sleep is similar to the adult REM sleep in that it is characterised by eye and other kinds of movement; however, unlike adults in REM, infants tend to enter AS at the beginning of their sleep cycle, as opposed to the end of it like REM in adults. Infants spend about half their time in AS/REM and half in QS, a much higher proportion than adults, who only spend about a quarter of their time in REM. It's hypothesised this difference in sleep pattern is related to the shorter sleep cycles and more frequent waking of infants. By 3 months infants become more likely to enter quiet sleep (or NREM, not-REM) at the beginning of their sleep cycle. After 3 months, infants tend to alternate between AS/QS for 50 minutes duration, as opposed to the longer cycle in adult sleep (90 minutes). Sleep cycle duration starts to resemble adult sleep more at 6 months, but doesn't fully resemble adult sleep until around 3 years old, which is generally around the time napping ceases as well. Frequent night waking and the short sleep cycle in infants is thought to be adaptive. Because infants have small stomachs and are undergoing rapid growth, they need to eat very frequently in order to get enough nutrition. However, because frequent waking and feedings interfere with parental sleep, this places the needs of the infant in conflict with the parents'. A review of sleep interventions found that sleep interventions done prior to 6 months  of age are ineffective and carries risk of harm. After 6 months, sleep interventions i.e. sleep training may improve maternal and infant sleep quality. Both breastfeeding and parental presence (i.e. rooming in or cosleeping) are correlated with increased night wakings, though some evidence suggests breastfeeding moms get more total sleep on average. The extent to which feeding type and sleeping arrangements influences parental sleep is controversial. References 

Infancy
Sleep