Have you ever wondered how our bodies know when it’s time to sleep? Although we might feel tired after a long day at work, or the day after partying well into the early hours, our bodies also have a clever internal rhythm to make sure we are prompted to go to sleep - circadian regulation.
In fact, it’s not just humans that use the 24-hour circadian rhythm to prompt activity through cycles of light and darkness (day and night). Most mammals use the circadian rhythm as a measure of daily cycle, although with humans it often signals an internal inclination to sleep.
But you need to go beyond the circadian rhythm to understand the full sleep pattern, so here’s the sleep cycle explained.
Four stages of sleep
Sighing. Shuffling. Slobbering. Snoring… These may be some of the physical activities our bodies do when lying in bed at night, but do you know that a healthy sleep cycle is split up into complex stages?
There are four unique stages of sleep that each carry with them vital functions for a rounded, refreshing and satisfying slumber. The first few stages are often categorised as ‘quiet sleep’ due to brain activity, whereas the final REM sleep stage is often ironically called ‘active sleep’ or paradoxical sleep.
Sleep stage 1 - Awake
This may seem like a stage that shouldn’t be included in the sleep cycle but being awake at night or in the morning plays a fundamental part in the sleep process. For a start, if we are awake we can’t be asleep, so this stage is a natural starting and ending phase of the overall sleep cycle.
Being awake as you get into bed or waking up in the morning isn’t the only time you can expect to be awake during the four-stage sleep cycle. Brief awakenings also happen between the other stages of light sleep, deep sleep and REM.
But those who are lying awake and unable to get to sleep can build up a significant sleep debt. Staying awake for large portions of the night whilst trying to get to sleep can impact behaviour, cognition and ability to function the next day.
Being awake is sometimes also known as ‘wakefulness’. Wakefulness is characterised by consciousness and can be recorded by the electrical activity of the human brain. When we’re awake we also have the conscious ability to perform tasks, which we can’t do when we’re asleep.
Although sleepwalkers can seem to be awake during their nighttime antics, sleepwalking is a feature of deep sleep and usually happens within the first couple of hours of falling asleep.
This phase of sleep is actually encountered during Non-Rapid Eye Movement (NREM) sleep. There are three stages within NREM sleep but the first two parts of NREM sleep are about the period between being awake and falling asleep.
Sleep Stage 2 - Light Sleep
When someone drops into a light sleep after sleep cycle stage 1, their brain's electrical activity slows down, along with breathing. This is a fuzzy period between wakefulness and sleep. If someone is woken up at this stage, they might say they weren’t really asleep, but instead ‘just drifting off’.
The transition between being awake and falling asleep in this period is usually about 5 to 10 minutes. In these ten minutes or less, our bodies are essentially trying to switch off from day mode to night mode, but unfortunately, unlike our electronic devices, there is no simple button to press for an easy switch.
The second stage of NREM goes even further into the sleep cycle as people start to experience light sleep. At this stage, body temperature will have dropped and people become less aware of their surroundings.
In this phase, it is not uncommon to experience muscle twitches, giving you the feeling of waking yourself up with a spasm. This type of twitch is known as a hypnic jerk and is a similar involuntary muscle movement to hiccups.
Sleep stage 3 - Deep sleep
This stage is so important to the sleep cycle we named our newest natural sleep supplement after it, Sleep Deep, to encourage a better night’s sleep for those who struggle to sleep soundly.
Flat out, out like a light or firmly in the land of nod, sleep stage 3 embodies the ‘deepest’ part of the sleep cycle, but is also, arguably, the most vital. When someone is experiencing ‘heavy sleeping’, or they aren’t woken up by noise or any changes to their environment, it’s probably because they’re firmly into the deep sleep phase.
Deep sleep is the final stage of NREM sleep, where breathing, muscle activity and heart rate are at their lowest across the entire sleep cycle. But your brain is still active, trying to form connections and memories or fresh things you’ve learnt. If you’ve had a mentally exhausting day learning new things, your brain will be working hard during sleep stage 3 to retain information.
Researchers have found that deep sleep could be an important factor in glucose metabolism and hormone release. Our bodies release growth hormones during deep sleep, which is crucial to repairing muscle and tissue, and for bone health too.
Stage 4 - REM Sleep
Your body repairs itself during REM sleep too, but it also promotes brain activity at an intense scale. The midbrain and forebrain experience heightened activity so it makes sense that REM sleep is most closely associated with vivid dreaming.
Is a sleep cycle 90 minutes? Well, we know that REM sleep starts around 90 minutes after first falling asleep. So, it’s possible you can go from watching a James Bond film to dreaming you’re 007 in the time it takes for The Living Daylights to end.
Although body temperature and irregular breathing are apparent in this stage, our bodily motor functions are reduced significantly to stop us from acting out our dreams.
Due to increased brain activity and rapid breathing, movement in your diaphragm increases and your eyes dart about behind your eyelids - hence the name Rapid Eye Movement sleep.
With all this extra brain activity during sleep, the REM stage of the sleep cycle has been coined as paradoxical sleep. This is because although our bodies are immobilised so that we don’t endanger ourselves by carrying out our dream escapades, our brains are awake with activity.
The numan take
The four stages of sleep and their various sub-phases each play an important role in the overall sleep cycle. During sleep stages 1 and 2, we’re transitioning from being awake to asleep. During the third sleep stage of deep sleep, we are at the most relaxed with the least brain activity. When we move into the more active REM sleep, our brains are a hive of activity that promote dreaming and cognition.