What is a sleep cycle and why does it matter?

23 Jun 2019

Even if you’re not much of a dancer, you’ve definitely got rhythm(s).

Our days and our bodies run on all sorts of cycles, such as the circadian rhythm – a roughly 24-hour body clock that tells us when we should sleep and wake. And there are a number of ultradian rhythms, which run through the course of one day, such as peaks and troughs in energy and focus. So it shouldn’t come as a surprise that sleep itself follows a pattern of stages and cycles – the kind you see mapped out in sleep tracking apps. And they do matter when it comes to sleep quality.

Over the course of the night, you should complete around five cycles of sleep (it can be four or six for some sleepers). Each one takes around 90 minutes, which is why you’re advised to give yourself a solid eight hours to sleep properly.

Each sleep cycle includes the same types of sleep and they’re all equally important – but they’re not actually identical to each other.

Stages of sleep

Within each cycle, there are four stages of sleep. Three of them are called non-REM sleep, and the fourth is, well, REM sleep. You might be familiar with the phrase REM; if not, it stands for Rapid Eye Movement.

Non-REM sleep is sometimes referred to as ‘deep’ sleep. It’s actually a three-stage process that gets deeper as it goes.

• Stage 1: The lightest non-REM stage. This is the bit where you’re drifting off. Occasionally, you might be aware of yourself suddenly jerking your arms or legs, or feel a falling sensation. Your eyes do move, but slowly.
• Stage 2: Initial non-REM sleep. Eye movements stop, and your body temperature and heart rate gently drop.
• Stage 3: Deep non-REM sleep. This is when it’s hardest to wake you and your brain waves are slow, delta waves. You can still move your muscles, so if you’re a wriggler – or sleepwalker – this is when it’s most likely happening.

Stage 4 is then REM sleep. It’s called that because of the quick movements of your eyes under your eyelids, and it’s the stage where you dream. Because your mind is busy – and your brain waves behave almost as if you’re awake – the muscles that move your body are held still. This is for your own safety, as you don’t want to act out your dreams without full control over your body – no matter how amazing they are!

Progressive cycles

In the early part of the night, your sleep cycles will favour more non-REM sleep. Closer to waking, you’ll get more REM sleep. That’s why it’s not enough to just get two or three cycles at a time – you need to get all your necessary stages within each 24 hour cycle.

Many experts think that the changing nature of the cycles means there are some progressive processes happening; for example, in our podcast episode on creativity with Sir John Hegarty, we discussed a theory from scientists at  Cardiff University that suggests the different sleep stages work together to develop problem solving. Each type is also associated with different forms of memory; non-REM sleep is thought to help us remember facts and information, whereas REM sleep is associated with remembering how to do something – like ride a bike.

Getting on track

There’s a lot more complexity to each cycle, of course. But the most important thing to take away from this is that if you’re routinely cutting short your sleep – or burning the candle at both ends – you’re likely to be missing out on part of a cycle that’s doing you good.

You can encourage yourself to get the full nights you need by tracking your cycles; our Simba Sleep App can do it from your phone, without the need to wear a tracker if you don’t want to. And you’ll be able to build a profile of your typical sleep style and get some tips for getting better sleep, too.