Everybody experiences both light and deep sleep every night. Here we look into Non-REM and REM sleep stages defined, plus the explain the sleep cycle.
The importance of sleep
Sleep is crucial for maintaining and improving physical and mental health. When we settle down and fall asleep each night, our body goes through different stages of sleep. There are two main types of sleep: non-REM (NREM) sleep, which is deeper sleep, and rapid eye movement (REM) sleep, which is lighter sleep. We all experience these two types differently, depending on various lifestyle factors including age, and also health factors such as stress levels.
How long does each stage of sleep last?
There are not only two types of sleep, but also different stages of each type of sleep. Specialist sleep psychotherapist Heather Darwall-Smith describes them below.
The first stage of sleep that we enter when we fall asleep is non-REM (NREM) sleep. It is divided into three stages:
Stage 1 – This is the lightest stage of sleep and usually lasts only a few minutes. It’s quite wakeful but you are technically under the sleep barrier.
Stage 2 – This stage lasts for about 20-30 minutes.
Stage 3 – This is the deepest stage of sleep, also known as slow-wave sleep, and is important for rest and recovery. During this stage, your brain waves slow down, and it is difficult to wake up.
The second stage of sleep occurs after the NREM stages, rapid eye movement (REM) sleep, and is characterised by rapid eye movements, increased brain activity, and vivid dreams. REM sleep usually occurs about 90 minutes after falling asleep and recurs several times throughout the night, getting longer with each cycle. During REM sleep, the body is paralysed to prevent acting out dreams, and breathing and heart rate may become irregular. The brain is active and uses this time to consolidate memories, process emotions, and repair the body.
What happens to the body during the stages of sleep?
As your body progresses through the sleep cycle stages, it transitions through different biological processes that affect your temperature, breathing, cells, and muscles. All the while, your brain is busy forming, organising, and storing memories.
Each sleep stage has a unique function and role in maintaining your brain’s overall cognitive performance. Some stages are also associated with physical repairs that keep you healthy and get you ready for the next day. There are various changes in the body during the different stages of sleep:
NREM Stage 1 Sleep
The first stage of the sleep cycle is a transition period between wakefulness and sleep. During stage 2 sleep:
- Your brain slows down
- Your heartbeat, your eye movements, and your breathing slow with it
- Your body relaxes, and your muscles may twitch
This brief period of sleep lasts for around five to 10 minutes. The brain is still relatively active and producing high amplitude theta waves, which are slow brainwaves occurring primarily in the brain’s frontal lobe.
NREM Stage 2 Sleep
People spend about half of their total sleep time during NREM stage 2, which lasts for about 20 minutes per cycle. During stage 2 sleep:
- You become less aware of your surroundings
- Your body temperature drops
- Your eye movements stop
- Your breathing and heart rate become more regular
The brain also begins to produce bursts of rapid, rhythmic brain wave activity, which are known as sleep spindles. They are thought to be a feature of memory consolidation—when your brain gathers, processes, and filters new memories you acquired the previous day.
NREM Stage 3 Sleep
Deep, slow brain waves known as delta waves begin to emerge during NREM stage 3 sleep – a stage that is also referred to as delta sleep. This is a period of deep sleep where any environmental noises or activity may fail to wake the sleeping person. During stage 3 sleep:
- Your muscles are relaxed
- Your blood pressure drops, and breathing slows
During this deep sleep stage, your body starts its physical repairs. Getting enough NREM stage 3 sleep makes you feel refreshed the next day.
While your brain is aroused with mental activities during REM sleep, the fourth stage of sleep, your voluntary muscles become immobilised. During REM sleep, your brain’s activity most closely resembles its activity during waking hours. However, your body is temporarily paralysed – a good thing, as it prevents you from acting out your dreams. During REM sleep:
- Your brain is active
- Your body is relaxed and in a form of paralysis
- Your breathing is fast and irregular
- Your eyes move rapidly
- You dream
Your brain also uses this time to cement information into memory, making it an important stage for learning.
Factors that affect your sleep cycle
Any time you have trouble falling asleep or staying asleep at night, your sleep cycle will be affected. Interrupted sleep is the term used to describe sleep that is not continuous throughout the night. When this happens, your sleep cycle can be disrupted. An in-progress sleep stage may be cut short, and a cycle may repeat before finishing.
Some factors that are associated with interrupted sleep, and therefore, may affect your sleep stages include:
- Older age – sleep naturally becomes lighter, and you are more easily awoken.
- Sleep disorders – including obstructive sleep apnea (breathing that stops and starts during sleep) and restless leg syndrome (an intense sensation of needing to move the legs).
- Pain – difficulty falling or staying asleep due to acute or chronic pain conditions.
- Lifestyle habits – little or no exercise, excessive caffeine intake, excessive alcohol use.
Tips for a healthier sleep cycle
If you practice good sleep hygiene, you can often improve the quantity and quality of your sleep. There are things everyone can try to help improve the quality and quantity of sleep:
- Limit electronics before bed.
- Try to get at least half an hour of natural sunlight per day.
- Go to bed and wake up at roughly the same time each day.
- Get some exercise every day.
- Don’t eat heavy meals before bed.
- Avoid alcohol.
- Keep your room cool and dark.