Understanding sleep: 4 stages of sleep and sleep cycle

We spend one third of our lives sleeping. Sleeping is undoubtedly important for our wellbeing.

And when we talk about the quality of our sleep, we normally focus on the duration (i.e how many hours) of sleep we are getting each night. While the duration of sleep is definitely important, it’s only part of the equation.

Have you ever wondered how sometimes you wake up feeling refreshed despite having just 4-5 hours of sleep but sometimes you wake up feeling sluggish even though you have had 6-7 hours of sleep? To understand why this happens, you first need to understand the different stages our brains go through in sleep.

Sleep cycle explained: What happens when we are sleeping

When we sleep, our brain goes through several rounds of sleep cycles, and each sleep cycle is comprised of four individual stages (awake, light sleep, deep sleep, REM sleep). On average, one sleep cycle lasts about 90 minutes and on average, we go through four to six sleep cycles in a typical night.

picture depicting sleep, sleep cycle and sleep stages

A typical sleep cycle goes like this:

• In the first 1 -5 minutes, we will enter stage 1 or N1 sleep stage. This is essentially the dozing off stage, our brain activities start slowing down, our bodies start to relax
• We will then enter the second stage of sleep, called stage 2 or N2 sleep. It’s harder to be woken up in this stage. Stage 1 and stage 2 sleep are also sometimes known as light sleep.
• If uninterrupted, we will then enter the third stage of sleep. Stage 3 sleep is the beginning of deep sleep, our muscle tone, breathing rate and pulse are all slower when we are in this stage.
• Finally, we will enter the REM sleep phase. REM stands for rapid eye movement. During this stage, our brain activities pick up and most vivid dreams happen in this stage.
• The cycle is completed and we go into light sleep again.

We will go through each sleep stage in more detail in the next section but right now, you need to know that every stage of sleep is important on its own. A cycle is completed when our brains go through all the 4 stages of sleep. Most of the time, regardless of how long you have slept, if you wake up in the middle of a cycle, you will wake up feeling terrible.

Sleep stages: REM sleep vs Non-REM Sleep

Before we get into the details of each sleep stage, let’s first understand the two main types of sleep we experience every night: REM sleep and Non-REM (NREM) sleep.

REM stands for rapid eye movement. During REM sleep, your eyes move around rapidly in a range of directions, but don’t send any visual information to your brain.That doesn’t happen during non-REM sleep.

So, every night when you are sleeping, your brain is actually cycling between REM and NREM sleep. It always starts with the NREM sleep and followed by REM sleep and back to NREM and so on.

What happens during NREM Sleep Stage: Light vs Deep sleep

Stage 1/N1 sleep stage (Light sleep, NREM)

This is the stage where you’re just falling asleep or “dozed off”.

During this stage, your body is still not completely relaxed yet, you can still hear what’s going on around you at this stage.

Your brain activities start to slow down, and your body might twitch occasionally and randomly. These twitches of your involuntary muscles are also known as hypnic jerks. The cause of hypnic jerks is not known yet but we do know that it occurs most frequently during stage 1 and 2 of the sleep cycle. Hypnic jerks are normal and not dangerous.

Your brain starts producing alpha and theta waves at this stage. Alpha brain waves are normally found in brains of people who are meditating or starting to relax, and theta waves are associated with memory, emotions and activity in the limbic system.

N1 sleep stage typically lasts about 5 minutes. It’s easy to wake someone up during this sleep stage, but if a person isn’t disturbed, they can move quickly into stage 2. 

Stage 2/N2 sleep stage (Light Sleep, NREM)

During stage 2, your body enters a more subdued state – body temperature starts to drop, muscles relax further, breathing and heart rate both begin to slow down.

Your overall brain activities slow down significantly with short bursts of activities – a phenomenon known as the sleep spindles. It’s believed that sleep spindles help us resist being woken up by external stimuli and improve our conversion of short term memory to long term memory.

Stage 2 sleep can last for 10-25 minutes during the first sleep cycle, and each N2 stage can become longer during the night. Collectively, a person typically spends about half their sleep time in N2 sleep.

N1 and N2 sleep stages are collectively known as the light sleep stage.

Stage 3/N3 sleep stage (Deep Sleep, REM)

Stage 3 or N3 sleep stage marks the passage from light to deep sleep. During this stage, your muscle tone, pulse, and breathing rate decrease as your body relaxes further.

Your brain activity starts showing an identifiable pattern of delta waves. Delta waves are observed when individuals are in deep sleep or in a coma.

It’s believed that this stage is critical to restoring your body. During this stage, your blood flow to muscles increases, and growth hormone is released, promoting cell recovery and growth. Even though your brain activity is reduced greatly during this stage, there is evidence to show that deep sleep helps improve your creativity, memory, and critical thinking skills.

It’s harder to wake someone up if they are in this phase, but if you are woken up from this stage, you will feel disoriented and groggy (in other words, terrible!).

About 90 minutes after you fall asleep and stay in NREM sleep, you will then enter the fourth stage of sleep which is the REM Sleep stage.

What happens during REM Sleep Stage

REM sleep stage

Your brain activity picks up significantly during REM sleep, nearing levels seen when you’re awake. This is when you will experience intense, vivid dreams.

At the same time, your body becomes immobile. The voluntary muscles of the body — arms, legs, fingers, anything that is under conscious control — are paralyzed. This is to protect you from acting out of your dream.

Another major characteristic of REM sleep is the rapid movements of your eyes. Even though your eyes are closed, they can be seen moving quickly in random directions.

REM sleep is believed to be essential for boosting creativity, solidifying memory, and learning.

It functions to process and store the information you obtained in the day, and turn it into long term memory. This is also why your dreams can be so weird during this stage – it’s just your brain is trying to organize everything together.

Typically, you don’t enter REM sleep until about 90 minutes later. The first REM stage may last only a few minutes, but as the night goes on, later REM stages last longer – some can last for as long as an hour.

Summary:

A sleep cycle is consist of NREM and REM sleep. NREM sleep can be further broken down into light sleep and deep sleep.

A typical sleep cycle goes like this: light sleep (stage 1 and stage 2 NREM sleep) -> Deep sleep (stage 3 NREM sleep) -> REM sleep. We go through about 4-6 cycles everynight. Each cycle lasts for about 90 minutes and you will spend more time in REM sleep in each subsequent cycle.

How Much Deep, Light, and REM Sleep Do You Need?

Importance and benefits of light sleep

Light sleep is usually the default stage you enter before getting into deeper stages of sleep.

It doesn’t really provide much of health benefits, but it’s the only way we can proceed to the more restful next stages. In other words, it’s important to prepare us for entering deep and REM sleep. It is the point where your muscles relax, your body temperature decreases, and your brain waves begin slowing.

It’s best to wake up when you are in the light sleep stage. You will feel refreshed and energetic if you are woken up from this stage.

How much light sleep do you need?

There isn’t a minimum hours of light sleep you need to get every night. Ideally you would want most of your sleep (>50%) spent in the deep sleep stage or REM sleep stage as these two stages are the “restorative” stages for your body and brain to recover.

If you are spending a lot of time in the light sleep stage, and not enough in the deep sleep or REM sleep stage, you will still get sleep deprivation symptoms.

Importance and benefits of deep sleep

Deep sleep is also known as the physically restorative stage. Many important processes happen during deep sleep.

During deep sleep, your muscles and tissues repair themselves and cells regenerate. This is also the stage when your body produces 95% of human growth hormone.

Other benefits of deep sleep include:

• increasing blood supply to muscles
• promoting growth and repair of bones
• strengthening the immune system – this explains why your immune system goes down when you are deprived in sleep.

How much deep sleep do you need?

According to Michael Grandner, the director of the Sleep and Health Research Program at the University of Arizona in Tucson, deep sleep is very much about the body. The thinking parts of the brain are largely offline and your body does a lot of rebuilding and repairing.

Around 13 to 23% of your sleep time is spent in the deep sleep stage. In other words, for an average person who sleeps 7 hours a day, about 1-2 hours are spent in the deep sleep stage.

And acccording to Grandner, there’s no real way to get too much deep sleep. Your body has its own natural drive for it, so once you meet that, the need will dissipate and you’ll just start going into REM and light sleep

The need for deep sleep decreases with age. Younger people need more deep sleep to promote growth and development whereas older people can get less deep sleep.

Importance and benefits of REM sleep

If deep sleep is about body, REM is about the brain,

Michael Grandner, MD, director of the Sleep and Health Research Program at the University of Arizona in Tucson 

REM sleep is the mentally restorative stage of sleep. When you are in this stage, your brain converts short-term memories made during the day into long-term memories. REM sleep is also found to be important for brain development – this explains why newborns spend most of their sleep time in REM.

Finally, REM sleep helps prepare us to wake back up from sleep by activation of our central nervous system. In other words, we are easier to wake up when we are in the REM stage. This is probably why we spend more and more time in REM stage as the night progresses.

How much REM sleep do you need?

There is no official consensus on how much REM sleep you should get.

For most adults, REM takes up about 20 to 25 percent of sleep, and this seems to be healthy during average sleep cycles.

Some actionable tips: