At every age, sleep involves cycles of two different types of sleep, which are predominantly determined and distinguished by the brain activity measured by the EEG. These two main types of sleep are known as Rapid Eye Movement sleep, or REM sleep, and non-Rapid Eye Movement sleep, or non-REM sleep. When we look at the EEG during REM sleep, we see low amplitude and mixed and high frequency waves, so waves that a small and fast, which are actually similar to the EEG when we are awake.
However, what is very different during this stage of sleep is the recurrent bursts of rapid eye movements, from which the name REM sleep is derived, as well as the presence of muscle atonia, or the complete loss of muscle tone. The rapid eye movements seen during this phase are not constant, or what we refer to as tonic, but they’re actually intermittent, or phasic, meaning that the EEG and muscle tone is changed throughout, but the bursts in eye movements are only seen for a portion of REM sleep period. In contrast to this, non-REM sleep consists of several different stages that have characteristically different brain activity patterns.
These stages is known as N1, N2, and N3, and also representative of sleep depth, with N1 sleep typically considered as slight sleep and having a low arousal threshold, meaning it is easy to be working up from this stage, and N3 representing deep sleep, and having a much higher arousal threshold, meaning it is much more difficult to be woken up from this stage of sleep than any other stage. The way in which these different stages of sleep occur undergoes changes across the lifespan, particularly in the early years of development.
In newborns, sleep occurs quite sporadically because they do not yet have fully functional circadian or body clocks, and their sleep tends to be split up across the course of the day or night, largely dictated by the need to feed. This type of sleep is termed polyphasic sleep. After a few months, periods of sleep become more well-defined and start to follow a more cyclical structure. And between the ages of three and five years, sleep transitions into what is termed monophasic, meaning it predominantly occurs once a day without the regular occurrence of naps.
For older infants and toddlers, the different stages of non-REM and REM sleep alternate in cycles that last approximately 50 minutes, while for children and adults these cycles are much longer, tending to last for around 90 to 110 minutes, with numerous cycles occurring throughout the night. One other main difference between adults’ and children’s sleep is the distribution of N3, or deep sleep, across these cycles. In adults the amount of deep sleep is predominant at the start of the night, tending to progressively decline across each subsequent cycle. However, although research in infants is limited, it appears that deep sleep follows quite a different pattern, and instead of declining through the night it occurs in every second cycle evenly throughout the night.
As we learned earlier, the amount of sleep, or sleep need, also changes dramatically across the lifespan. On average, infants need twice as much sleep, if not more, than adults do. Interestingly, the proportions of the different stages of sleep are quite different as well, with REM sleep and non-REM sleep each accounting for approximately 50% of the total sleep time. As we age and sleep less, the proportions of non-REM and REM sleep change dramatically. This reduction in sleep need results in us spending a lot less time in REM sleep, with non-REM sleep making up about 80%, and REM sleep accounting for about 20% of total sleep by the time we reach adulthood.