Basic Sleep Biology

by Consuela Hendriks

What is a sleep cycle?

Let’s talk som basic sleep biology!


We all sleep in cycles. Such sleep cycles differ in length depending i.a. on age.

During a sleep cycle we go through different sleep stages. The sequential order (and length) of sleep stages is called sleep architecture. These stages do not appear randomly but are rhythmic, repetitive patterns throughout the night.


A typical sleep cycle looks like this:


We enter sleep through Non Rapid Eye Movement sleep.


NREM consist of 4 distinct stages:

  • NREM1 is the transition stage: you enter light sleep from a state of wakefulness. During this stage auditory and visual perception decreases but it’s still easy to wake up (low arousal threshold) . There can be jerking movements of the body (hypnic myoclonia) and visual imaginations (hypnogogic hallucinations).
  • Then we enter NREM2. During this light sleep stage there are sleep spindles and K-complexes, which are specific EEG activity episodes. These protect the brain from external stimulation, so they help to stay asleep and enter more deep sleep. They also play a role in memory consolidation.
  • NREM3 (previously seperated into NREM 3 and NREM4) is next, also known as slow wave sleep or delta sleep. In this deep sleep stage delta wave activity is seen. This is the restorative sleep stage where cell growth and repair takes place and growth hormone is released. Respiration is slow and regular; blood pressure decreases and there is moderate muscle tone. There is a high arousal threshold, it hard to awaken.


After deep sleep you cycle back into lighter sleep and go into Rapid Eye Movement sleep. REM is where most (recalled) dreaming appears. The brain is very active (some parts more active compared to the awake state) but there is temporarily muscle paralysis (except for autonomic functions, ocular muscles, middle ear and erectile muscles). There are burst of rapid conjugate eye movements (two eyes move in the same direction). It is thought that this stage is important for i.a. memory consolidation, synaptic pruning and emotional health.


After REM sleep, at the end of each sleep cycle, there normally follows a brief arousal (a short awakening). These arousals aren’t always remembered.

Progression of sleep across the night

A sleep cycle looks different in the beginning of the night compared to the early morning.


Both NREM and REM sleep are under strong circadian control, meaning that they are linked to our body clock (Dijk, 2009).


As the night progresses the amount of deep sleep (NREM 3-4) decreases while the amount of REM-sleep increases.

So, the first sleep cycle contains relatively much deep slow wave sleep, and this diminishes during subsequent cycles.

The amount of REM sleep in the first sleep cycle is relatively short and becomes longer in subsequent cycles.


But besides (circadian) timing, the amount of SWS and REM are influenced by several other factors (Grigg-Damberger, 2016; Mindell & Owens, 2015; Ohayon et al., 2004):


What influences the amount of deep Slow Wave Sleep (SWS) sleep?

  • Prior wakefulness: The longer you’ve been awake, the more subsequent SWS you’ll have.
  • Age: there is an age-related decline in SWS. Infants’ and children’s cycles contain a relatively large amount of SWS.
  • Prior sleep: Following sleep deprivation or restriction, subsequent sleep will contain more SWS at the expense of light sleep (SWS rebound).


What influences the amount of REM sleep?

  • REM sleep is less affected by prior wakefulness.
  • Age: there is an age-related decline in REM: The proportion of REM sleep from birth decreases through early childhood into adulthood (at birth 50% – 6 months 30% – 20-25% by 5 years of age)
  • Prior sleep: Following sleep deprivation or restriction, subsequent sleep will contain more REM at the expense of light sleep (REM rebound).


So, in short, the amount of deep sleep and REM sleep depend on the time we sleep, prior (lack of) sleep, prior wakefulness and age.

Baby sleep differs from adult sleep

There are many differences in newborn sleep compared to adult (or older baby’s) sleep:

  • Sleep need: Newborns sleep more. Sleep needs decline by age.
  • Sleep cycle length: Sleep cycles in healthy term infants typically last a mean of 50–60 min (range, 30–70 min). Adult sleep cycles are roughly 90-100 minutes. Note that this is a range, shorter sleep cycles are normal too!
  • Sleep stages: Newborns have 2 main sleep stages:

Active sleep (similar to REM) and Quiet sleep (similar to deep NREM).

The state of sleep where the characteristics are not clearly defined into active sleep or quiet sleep is known as indeterminate or transitional sleep.

  • During Active sleep (REM) there is no muscle paralysis in newborns. Skeletal muscle atonia development is immature. This probably is because there is no need to protect the infant for acting out their dreams yet. You can often see movement of the eyes, twitching and jerking of fingers, irregular breathing , the baby pulling funny faces or smiling while in active sleep.
  • Newborns don’t enter sleep through NREM but through active sleep, until sleep architecture matures to more adult-like patterns (around 3 months)
  • A newborn’s sleep cycle typically consist of REM-NREM-REM.
  • Babies have more end-of-cycle arousals: They fully wake up more often after a sleep cycle compared to adults. This is because their sleep cycle length is shorter, but also because their ability to self-regulate and self-settle is not mature enough. They often need someone to help them co-regulate in order to drift off again. So, they do not sleep through the night.
  • Newborns also do not sleep through the night because their circadian rhythm isn’t fully developed. Adults also technically don’t sleep through the night; after each sleep cycle there is a brief arousal, we often just don’t remember these arousals.
  • Babies have more SWS and REM compared to adults.
  • The higher percentage of SWS also results in a higher chance of partial arousal parasomnias in childhood since parasomnias are SWS related. A parasomnia is a term for unusual and undesirable physical events or experiences that disrupt sleep. Examples of parasomnias are sleep terrors or sleep walking.
  • The higher percentage of REM sleep results in a higher chance of nightmares, since nightmares usually occur during REM sleep.


Dijk, D. J. (2009). Regulation and functional correlates of slow wave sleep. Journal of Clinical Sleep Medicine5(2 suppl), S6-S15.


Grigg-Damberger, M. M. (2016). The visual scoring of sleep in infants 0 to 2 months of age. Journal of clinical sleep medicine12(3), 429-445.


Mindell, J. A., & Owens, J. A. (2015). A clinical guide to pediatric sleep: diagnosis and management of sleep problems. Lippincott Williams & Wilkins.


Ohayon, M. M., Carskadon, M. A., Guilleminault, C., & Vitiello, M. V. (2004). Meta-analysis of quantitative sleep parameters from childhood to old age in healthy individuals: developing normative sleep values across the human lifespan. Sleep27(7), 1255-1273.