Although babies spend much of their time sleeping, this sleep doesn't happen only at night. Nor do newborn infants sleep all night.
Infant sleep is more or less evenly distributed between day and night, and occurs in 2-3 hour bouts (Gertner et al 2002). As infants' circadian rhythms (day/night cycles of hormone production etc) develop, sleep gradually becomes concentrated during night-time hours (Parmelee, Wenner and Schulz 1964).
During sleep we pass though periods where our conscious brain is active and processing information (known as REM [Rapid Eye Movement] sleep or active sleep) and periods where our conscious brain is quiet and 'resting' (known as Quiet Sleep). REM sleep is when we dream. Quiet sleep is considered to be restorative sleep, during which we are 're-charging' both brain and body. When adults sleep, we fall quickly into Quiet Sleep, and later move into REM sleep. We experience 'sleep cycles' of 90 minutes each; these begin with Quiet Sleep and end with REM sleep. At the beginning of the night each cycle has mostly Quiet Sleep with a small amount of REM. Towards the morning each cycle contains less Quiet Sleep and more REM; we therefore dream more towards the morning than at the beginning of the night. Overall adults experience 20-25% of our sleep as REM and 75-80% as Quiet Sleep.
Babies are not little adults!
Babies are very different, and to begin with their brain activity differs from adults during sleep, so REM sleep for babies is called 'Active Sleep'. During gestation babies' sleep is mostly Active/REM sleep. After birth Active/REM sleep remains dominant (>50%), and when babies fall asleep they fall first into Active/REM sleep (which means they can easily be awakened during this period). After about 20 minutes they drop into Quiet Sleep (this is the 'floppy baby' stage of sleep when they can easily be moved without being disturbed). Babies' sleep cycles are about 60 minutes long, and are mostly composed of Active/REM (their brains are very active during sleep, and during this early phase of life their brains are growing rapidly with new neural connections being made constantly). (Peireno, Algarin and Uauy 2003; Anders et al. 1995, Tarullo et al. 2010)
As babies mature the amount of sleep time spent in the Active/REM state reduces, and the amount spent in the Quiet Sleep state increases. When they reach adult proportions of REM and QS, this is technically known as Sleep Consolidation. As REM sleep reduces, particularly during the daytime, wakefulness increases (Coons and Guilleminault, 1982).
Babies do not all sleep in the same way.
In the period before sleep becomes consolidated, behavioural or observed sleep-wake patterns can vary considerably between infants. Why some babies appear to sleep through the night and others wake frequently is still not fully understood.
Research using infrared video recording have provided evidence that, although sleep periods lengthen with age, infants continued to wake up during the night during the first year of life. St James-Roberts et al’s (2015) research used video and parental questionnaire data to examine one hundred infants’ sleep patterns at 5 weeks and 3 months of age. They found that a quarter of infants woke and resettled themselves at night, most often without parental awareness. St James-Roberts et al concluded that ‘sleeping through the night’ involves both prolonged bouts of sleep alongside episodes of autonomous resettling.
A study in Japan (Yoshida et al. 2015) found that observing babies' behaviours may only tell part of the picture. In this study some infants who were not showing behavioural signs of being awake were in fact, according to their brain wave activity, awake but not moving. This same study suggested that infant feeding method influences the quality sleep experienced by infants, although St James-Roberts et al found that feeding method did not influence babies’ ability to resettle themselves to sleep, and infants fed human milk were as likely to self-resettle as infants fed formula or mixed human and formula milk. It is, therefore, important to consider which aspects of sleep we are being considered, and the role of other factors on each of these components of infant sleep, such as, variations in parenting style, environment, socio-cultural practices and evolved biology. These issues are discussed in the pages that follow.