NREM sleep (standing for non rapid eye movement) is the first type of sleep you enter into when you first fall asleep at night. It makes up 75% of an adults total time spent asleep. NREM sleep is divided into three stages, going from the lightest stage of sleep to the deepest stage of sleep.
Each stage of NREM sleep is classified by its unique brainwave patterns as displayed by an EEG (electroencephalogram) machine using small round electrodes dotted around the head. These brainwaves are so small that they need to be magnified by over a million using specialist machinery for us to be able to view them.
Your brainwaves get slower as you relax, fall asleep, and enter the progressively deeper levels of NREM sleep. When we’re in a state of relaxed wakefulness our brainwaves are in the alpha state, measuring between 8 to 12 Hz (cycles per second).
NREM Stage 1 – The Lightest Stage of Sleep
Stage 1 is denoted when your brainwaves go from the alpha state to the theta state, measuring between 4 to 7 Hz. It’s an extremely light and hazy type of sleep that happens when your muscles relax and your mind starts to drift off.
People in this stage of sleep can be very easily woken up, just my making a noise or turning on a light. Some people who have been woken up from NREM stage 1 don’t claim that they had actually fallen asleep but instead claim to have had a momentary lapse in concentration. NREM stage 1 is where microsleep occurs, a very short sleep that lasts for less than 30 seconds.
As the muscles relax, you might experience a sudden jolt combined with a feeling of falling. These are called hypnic jerks. No one really knows why they occur but they’re completely normal.
NREM stage one lasts for around 10 minutes, but can last much longer for people who have difficultly falling asleep. Many people who claim to have stayed awake in bed most of the night spend a good proportion of their time here, where they have technically been sleeping but so lightly that they didn’t even notice it. But generally, around 5% of our total time asleep is spent in this stage.
NREM Stage 2 – Drifting Deeper into Sleep
Stage 2 is often considered the beginning of proper sleep. Since sleep is such a gradual process, it’s actually quite hard for researchers to properly define when someone has fallen asleep. There’s a very fine line between relaxing deeply and having fallen asleep.
It’s still a reasonably light stage of sleep. You could still quite easily wake someone up from stage 2, but unlike stage 1 they would know that they had just been sleeping.
The key signs that show someone is in NREM stage 2 are:
- Sleep spindles – A burst of fast waves lasting for less than a second
- K-Complexes – A single long delta wave that lasts for just a second
It’s believed that sleep spindles and k-complexes are used by the brain to block out any harmless distractions and keep you sleeping.
In this midway stage of sleep your body begins to prepare your body for the deep sleep ahead. Your heart rate slows, temperature drops and your brainwaves become slower. Your eyes are still and your muscles completely relaxed.
Adults spend around half their sleep time in NREM stage 2. It lasts for around 10 to 20 minutes.
NREM Stage 3 – The Deepest Stage of Sleep
The replacement of theta waves with long delta waves of 1 to 3 Hz represents the entry into NREM stage 3. This is the deepest stage of sleep, known as slow wave sleep or deep sleep. In this stage your brainwaves are at their longest and slowest.
It’s very hard to wake someone up from deep sleep. Even if you did, they would feel very groggy, disorientated and not very alert.
NREM stage 3 is where sleepwalking and night terrors occur. Despite appearances, people with these sleep disorders aren’t playing out a dream. They’re fast asleep and most likely won’t remember anything of the episode, or if they do it would only be distant fragments of dreams.
20% of our time is spent in deep sleep, which lasts for up to an hour.
How deep our sleep becomes in this stage depends on:
- The length of time without sleep – The longer you have stayed awake for, the deeper your sleep will be
- Stimulation of the brain during the day – In particular visual stimulation from different surroundings
- Age – Older people tend to experience less deep sleep than younger people
Many different drugs and substances such as alcohol can also prevent us from entering the deeper stages of sleep at night.
NREM Stage 4?
Stage 3 (deep sleep) was originally split into two separate stages known as stage 3 and stage 4. They were classified as follows:
- Stage 3 – When delta waves first started to appear within the theta waves
- Stage 4 – Where more than 50% of the brainwaves were delta waves
So stage 3 was originally a 5 minute stop gap in-between stage 2 and 4 signaled by the arrival of the first delta waves.
There wasn’t much difference between these two stages and many sleep professionals referred to them collectively as deep sleep anyway. So in 2007 the American Academy of Sleep Medicine combined the two, but traditional four stage approach is still widely used around the world.
The NREM Sleep Cycle
The NREM stages of sleep help us map out what happens to us when we fall asleep.
- Stage 1 – Relaxed alpha waves turn into slow theta waves as we feel ourselves drift off
- Stage 2 – The theta waves become slower and we become fully asleep
- Stage 3 – The theta waves are replaced by long slow delta waves as we enter deep sleep
But after stage 3 something remarkable happens. The brainwaves become rapidly shorter, almost looking like we have woken up, but from appearances it looks as though we’re still fast asleep.
We enter into a completely different type of sleep known as REM sleep (rapid eye movement). We stay in this stage for around 10 minutes before going back into NREM sleep, starting again at the lightest and going progressively deeper.
These cycles of NREM and REM last for around 90 minutes before repeating themselves again through the night. As the night continues, NREM becomes less deep and we stay in REM sleep for longer.
To see a graph of the stages of sleep we enter into at different times during the night, see the stages of sleep article.