Sleep is a fascinating subject that holds the key to many questions about how our bodies function, heal, and restore themselves. We know that without it, our body’s can start to show signs of wear and tear, increased stress levels, as well as health problems.
So what is the deepest stage of sleep and which is the lightest phase of sleep? At what stage do you dream? Which is the most restful or restorative sleep stage? In what stage of sleep does sleepwalking occur? Researchers have worked to answer all of these questions down the years. Here’s what goes on in our mind and body when we catch some shut-eye!
Understanding Sleep Cycles
When you fall asleep you experience two kinds of sleep – Rapid-Eye-Movement (REM) sleep and Non-Rapid-Eye-Movement (NREM) sleep. You alternate between these two several times during the course of your sleep.
REM happens when you’re sleeping less deeply and is accompanied by rapid eye movements – something so typical of it that this sleep stage is named after it. REM is the “dream phase” as it is believed to be closely linked to dreaming. People roused from this sleep stage usually remember their dreams or recall having a dream. In fact, the muscles of your legs and arms are paralyzed temporarily when you experience REM sleep. It is theorized to be your body’s defense system to prevent you from trying to act out your dream!
NREM sleep phases tend to be those of deep sleep and is marked by reduced physiological activity.1 Your breathing, heart rate, and blood pressure slow down in this stage. NREM isn’t considered a dream phase as very few people are likely to remember they had a dream when they are woken from NREM. Interestingly, the amount NREM sleep you get is said to increase as you grow older.2
Through the course of your sleep, you go through a sleep cycle which has multiple stages of NREM and REM sleep. These stages are marked by different levels of brain activity, measured in sleep stage waves.
What Are Sleep Stage Waves?
The waves marking the level of activity in your brain are an important piece in the whole “what happens when you sleep” jigsaw. So here’s the lowdown!
- When you’re awake, your mind is buzzing with activity. At this stage, your brain produces beta waves that are very high in frequency and of low amplitude. They also tend to be less synchronous or consistent in pattern.
- As you relax, the waves slow down and their amplitude rises, even as they become more synchronous. Such waves are alpha waves.
- The more the body relaxes, the slower the waves get – as in the theta waves of stage 1 of sleep. There are also delta waves and sleep spindles to reckon with – more on that later!3
How Is Sleep Tracked?
Researchers use recordings of electroencephalograms (EEGs) to track brain activity, along with measurements of muscle activity using an electromyogram (EMG) and eye movements with an electrooculogram (EOG). Sleep studies over time show that characteristic electrical patterns are formed by your brain when you sleep. Your eye movements also change depending on your sleep stage, becoming more rampant when you are in the REM sleep phases.
What Are The Different Sleep Cycle Stages?
So how many stages of sleep are there in all? Research indicates that there are likely to be five stages or phases of sleep in all.
- You begin with stage 1 sleep, followed by stage 2, 3, and 4 of NREM sleep before entering REM sleep.
- Once you complete one sleep cycle, you begin again with stage 1 sleep.
- A complete sleep cycle runs between 90 and 110 minutes.
- At first, you go through various cycles with less REM and more NREM.
- As you get closer to morning and waking up, you end up going through just sleep stages 1 and 2 followed by REM sleep.4
Stage 1 Sleep
The first stage, stage 1, or stage N1 of NREM sleep is a light phase of sleep when you can be woken up easily. Here’s what defines this first stage of sleep5:
- You don’t sleep very deeply and find yourself drifting in and out.
- Eye movement is slow.
- Muscle activity in your body also begins to slow down.
- You may feel a sudden contraction of your muscles and even a bit like you’re falling before this happens – this is called hypnic myoclonia.6
- If someone were to wake you now, you may be able to recall fragmented visual images.
- Sleep stage waves: Alpha and theta waves are produced by your brain.
- Duration of the first stage of sleep: 7 minutes on an average.
Stage 2 Sleep
About half of the entire time you spend asleep is spent in stage 2 or stage N2 of NREM sleep.7
- This is also a fairly light sleep stage.
- Eye movements stop in this stage.
- Sleep stage waves: Brain waves slow down, interspersed by occasional rapid waves – these are known as sleep spindles.8
- Duration of N2 stage of sleep: 10 to 15 minutes on an average.9
Stage 3 And Stage 4 or Stage N3 Sleep
Stage 3 and stage 4 of NREM sleep are together called “deep sleep.” Earlier, they were looked at separately but are now collectively called N3 sleep.10
Here is what you can expect in the N3 stage of sleep11:
- The deep sleep phase, also called short wave sleep, is the most restful and restorative stage of sleep.
- No eye movements happen in this phase.
- There is no muscle activity.
- It is hard to wake you up in this stage.
- If you do wake up, you may be disoriented for a while and drowsy/groggy.
- Sleep stage waves: Brain waves are extremely slow – these are delta waves. They are punctuated by some smaller faster waves in between. Once you’re in stage 4 sleep, the faster waves will cease and only delta waves remain.
- Sleep stage known for: Sleepwalking, bedwetting, and even night terrors among children occur during this deep sleep phase.
- Duration of N3 stage of sleep: The amount of time you remain in this stage of sleep is linked to changes in the central nervous system. It also depends on your accumulated sleep need. So if you have been awake longer, you need even more of this stage of sleep.12
REM Sleep: The Fifth Stage
After the four initial phases of NREM sleep, the body enters REM sleep, the final sleep stage before the cycle begins again. You spend around 20 percent of your sleep time in REM sleep. Here are some other characteristics13:
- Breathing is rapid, shallow, and irregular.
- Eyes jerk quickly in different directions.
- Your limbs are temporarily paralyzed.
- Heart rate rises.
- Blood pressure goes up.
- Some men may have a penile erection.
- If you are woken in the REM sleep stage, you may have the story of a fantastical, bizarre, and often illogical dream to tell.
- Sleep stage known for: Dreaming!
- Duration of REM stage of sleep: This sleep period happens anywhere from 70 to 90 minutes post your first sleeping off and lasts typically between 10 and 20 minutes or 20 percent of your sleep cycle. Your initial sleep cycle for the night has a shorter REM period and more deep sleep. But as you get closer to morning, REM sleep periods lengthen.
How Are Infant Sleep Stages Different?
Adults or older children go through five phases of sleep with half the time in stage 2 sleep, 30 percent divided between sleep stages 1, 3, and 4, and 20 percent in the REM sleep stage. Infants, however, spend 50 percent or half their entire sleep time in the REM sleep stage.14
References ↑1 Stages of Human Sleep. National Sleep Foundation. ↑2 Non-REM SLEEP. Prince George’s Community College Maryland. ↑3 Stages of Sleep. Psychology World, Missouri University of Science and Technology. ↑4, ↑6, ↑7, ↑8, ↑11, ↑13, ↑14 Brain Basics: Understanding Sleep. National Institute of Neurological Disorders and Stroke. ↑5 Understanding Sleep Cycles: What Happens While You Sleep. National Sleep Foundation. ↑9 Natural Patterns of Sleep. Division of Sleep Medicine at
Harvard Medical School.
↑10, ↑12 Stages of Human Sleep. National Sleep Foundation.