Normal Sleep Development
As any parent of a newborn infant knows, although babies spend much of their time sleeping, this sleep doesn't happen predominantly at night. Nor do newborn infants sleep all night.
In fact 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 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 very different. In pregnancy babies' sleep is mostly active/REM sleep. After birth REM sleep remains dominant (>50%), and when babies fall asleep they fall first into 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 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 REM reduces, and the amount spent in Quiet Sleep 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).