When you sleep at night, your body goes through 4 separate stages of sleep as you move through the night. Several decades ago it was previously assumed by scientists that when a person falls to sleep that their brain entirely shuts off to allow your body to dedicate all of its resources and energy to enable the body to recover.
However, since this time it has now been discovered that sleep is significantly more complicated than first thought and your brain spends a surprising amount of time in an active state throughout the night. Your brain goes through many different activity phases which can be categorised as Non-REM sleep and REM sleep.
We’ll outline the different phases of sleep and how each one affects your body to leave you feeling refreshed in the morning.
Stages of Sleep
NREM Stage 1
Stage 1 of non-rapid eye movement sleep (NREM) happens when you first decide to sleep and close your eyes. This stage usually lasts a maximum of 10 minutes, and throughout this stage, you are only in a light sleep which means that you can easily be disturbed and woken up.
If you’re woken up in this stage, you’ll often feel like you got no sleep at all. This is the beginning of your body slowing down which is usually characterised by your heart rate and breathing decreasing. Muscles also begin to relax as well as the brain entirely.
A common sign of stage 1 sleep is the hypnic jerk that we occasionally experience or the feeling that you’re falling. This sensation is claimed to be linked to an evolutionary safety trait for humans, so it is perfectly reasonable to experience this.
NREM Stage 2
Stage 2 of NREM is still recognised as a light sleep but continues the progression moving closer to deep sleep, and it’s harder for you to be awoken as easily as stage 1. Stage 2 sleep usually lasts for around 20 minutes, and brain waves continue to decrease in speed with precise periods of swift activity which are referred to as speed spindles.
Your heart rate remains at a slow speed while your body temperature also lowers. This substantially decreases the amount of activity your body does as it prepares to enter a deeper sleep. Also, other metabolic functions also slow down.
If you have a power nap, then you want to avoid entering a deep sleep so you would aim to wake up just after the end of stage 2 sleep and no longer than 30 minutes in total.
NREM Stage 3
Previously there used to be a stage 3 and 4; however, this has now been combined into one stage 3 sleep. This is the deepest stage of NREM sleep, and most body restoration occurs at this time with slow waves present.
The chances of being woken out of this phase are significantly lower than the previous stages, and this usually starts 35 to 45 minutes after you have fallen asleep. Results from studies using electroencephalograms have shown that brain waves slow down further but also become more substantial.
If you wake up within this stage, then you may feel groggy or disorientated at first. This is the feeling you get when you nap for slightly too long. Throughout this stage, the glymphatic system eradicates waste, such as beta-amyloid and neurotoxins from the brain.
REM sleep is commonly known as rapid eye movement sleep and is where you enter a dreaming state of sleep. REM sleep will occur after being asleep for around 90 minutes after cycling through the previous sleep stages. This is the final stage of a normal sleep cycle and as the name suggests, features rapid movement of your eyes from side to side.
Brain waves are considerably more active compared to sleep stages 2 and 3 which can cause intense dreams; however, awakenings are more common and can leave you feeling groggy. This stage of sleep typically lasts for around 10 minutes; however, this stage gets longer as the night progresses and the final REM sleep stage can last for up to an hour.
In terms of body functioning, throughout this stage, your blood pressure and heart rate will rise leading to breathing speeding up and being more irregular. It's the stage where your brain consolidates, processes and stores information that you’ve learned from the day into your long term memory, making it easier to recall in the future.
As a healthy adult, REM sleep typically accounts for 20-25% of the overall sleeping time, whereas NREM sleep stages 1-3 take up the remainder of your rest. So if you sleep for the recommended 8 hours per night, then this equates to around 1.5 to 2 hours of rapid eye movement sleep per night.
What happens throughout the night?
In reality, sleep isn’t as straightforward as you go through 4 to 6 of the 90 minutes sleep cycles that have been described. Throughout the night the number of minutes you spend in each sleep stage will vary depending on many factors.
Within the first 2-3 sleep cycles you’ll spend the majority of your sleep in stage 3 however when you reach the final 2-3 sleep cycles of the night more time will be spent in REM restorative sleep. The amount of time you spend in each stage is also linked to your circadian rhythm and the time that you’ve decided to go to sleep, to begin with.
Generally speaking, you’ll spend more NREM sleep during the opening hours of your sleep (10 pm to 2 am) and more time in REM sleep towards the later hours of your sleep (2 am to 7 am). It means that night owls can often obtain more REM sleep than those who are early settlers.
What is deep sleep?
You’ve probably heard of the term deep sleep, and this occurs in stage 3 of NREM sleep which is where the most body restoration takes place. This deep sleep is the most difficult to be woken out of, and if you find it challenging to be woken up even with a loud alarm clock, then it’s highly likely that you’re in a deep sleep when the alarm time triggers.
If you’re suffering from sleep deprivation, then you’ll spend more time in stage 3 than usual. Deep sleep lowers your sleep drive and is recommended you avoid if you take a nap throughout the day. If you fall into a deep sleep throughout the day, then you may find it more difficult to fall asleep at night because you’ve reduced your bodies need for sleep.
Human Growth Hormone is a crucial peptide hormone that stimulates growth and cell reproduction. It helps your body to repair your muscles and body from the previous day.
Factors that affect Deep sleep
The amount of time and frequency of deep sleep you experience each night can vary depending on many factors.
Younger infants spend more tie in deep sleep, but as you get older, the amount of deep sleep naturally drops. This is not only due to the body changing but also life pressures and responsibilities which restricts the amount of time you can spend in bed.
Sleep disorders such as sleep apnoea or snoring can interfere with the amount of deep sleep you get each night. If your sleep disorder frequently wakes you out of your sleep, then this decreases the quality of your sleep which can leave you feeling tired and unrested when you finally wake in the morning.
Your circadian rhythm is your body’s natural body clock, and it wants to sleep at night not throughout the day. If you work nights and try to sleep in the day, then at times you may need to force your body to sleep which can delay the onset of deeper sleep.
Cutting your sleep short
Life can throw all sorts of things at us so some nights it may not be possible to get the recommended minimum of 8 hours sleep per night. So obviously the less time you spend sleeping, the less deep sleep you may get. However, if you’re deprived of sleep long term, then this can lead to more severe effects on your body.
How to measure Sleep cycles?
While it isn’t essential to measure your sleep cycles, if you’re consistently waking up tired and unrested, then it’s likely that your sleep isn’t optimal. To remedy this, you will need to investigate how you’re sleeping including how long you’re spending in each sleep cycle.
With the improvement in modern technology, you no longer have to go to residential sleep studies, and you can now do this with a sleep activity tracker. These devices can track your heart rate and movement throughout the night while also estimating the amount of time you spend in light, deep and REM stages of sleep.
Each stage serves a different purpose for your body so having this data will allow you to understand and recognise any sleep related issues that may be present.
You can also compare and benchmark your stats to the averages of others who are the same gender and age range.