The key to getting good sleep is understanding how it works. But it’s a complicated thing. And, while we usually hyperfocus on sleep duration, counting the hours of rest we get and assuming the more the better, recent studies have shown that sleep quality is actually more important.
A key indicator of sleep quality is the way our sleep cycles look. A sleep cycle is made up of four main stages: REM, and NREM1, 2 and 3.
How long you spend in each stage, how quickly you transition between stages, and how many complete sleep cycles you experience determines how well rested you will feel in the morning.
But how do sleep cycles work? To find out, I took part in a polysomnography. This is a gold standard sleep study that helps diagnose disorders like sleep apnea. It monitors body and muscle movement, brain activity, heart rate and blood pressure.
With the help of Dr Alanna Hare, President of the British Sleep Society, and iPDiagnostics, a leading private healthcare provider specialising in home-based sleep and neurological diagnostic testing, I was able to find out exactly what a sleep cycle looks like, using my own data.
For Sleep Awareness Week 2025, I’m sharing my results and everything I learned about sleep cycles in the process.
What is a sleep cycle?
You may think your night comprises of a simple sleep/wake cycle, characterized by a sleeping state and a waking state. However, it’s a little more complex.
“There are two types of sleep: Non-rapid eye-movement (NREM) sleep and rapid eye-movement (REM) sleep,” Dr Hare explains. “NREM sleep is divided into stages 1, 2 and 3.”
REM sleep is when we dream, and NREM sleep includes light and deep sleep. Each have different roles in our brain and body’s functioning.
When we first fall asleep, we don’t experience REM sleep straight away. “An adult’s sleep episode begins with a short period of NREM stage 1 progressing through stage 2, followed by stages 3 and 4 and finally to REM,” Dr Hare tells us.
“Individuals do not remain in REM sleep the remainder of the night but, rather, cycle between stages of NREM and REM throughout the night.”
“Adult sleep is characterised by three to five cycles of NREM/REM sleep throughout the night, each cycle lasting around 90-100 minutes.”
Dr. Alanna Hare
Dr Allie Hare is an award-winning consultant in Sleep medicine. She is the President of the British Sleep Society and is the co-founder of Grace London. She runs clinics in Sleep medicine in the NHS and in the private sector on Harley Street and Wimpole Street. Her advice on sleep has been featured amongst others, in British Vogue, the Guardian, the Telegraph, the Robb Report, the Wall Street Journal, The Sunday Times and on the BBC. She works with a number of corporate organisations, supporting sleep wellbeing, and has advised athletes and sporting organisations, including Formula 1 teams.
How my sleep was monitored
I took part in a polysomnography sleep study, which is considered the gold standard of sleep studies and can be used to diagnose a range of health conditions. The study was conducted by iPDiagnostics and the results were analyzed by Dr Hare.
iPDiagnostics are able to conduct these sleep studies in the comfort of your own home, rather than a clinical environment like a hospital, so your results most closely resemble your natural sleep patterns.
I travelled to London and stayed in a hotel of my choice and the team came to my room to set up the study.
This meant attaching sensors to my scalp to pick up brain activity, linking me up to heart rate monitors, attaching eye movement and chin movement sensors to my face, placing a tube in my nostrils to measure oxygen levels, clipping a blood pressure monitor to my finger and sticking sensors to my legs to measure limb movement. A video camera was also set up to record any unusual activity.
The team were then able to identify any underlying sleep disorders, observe my full sleep cycles and record any abnormalities in my sleep.
It would also show any parasomnias (like night terrors, sleep talking and sleep walking,) which was my initial complaint.
This process took around an hour, but luckily all the sensors were strapped to my body, rather than to anything external.
This meant I could move around freely and enjoy a calming nighttime routine without feeling like I was connected or strapped to anything.
In the morning, the team came to disconnect me, and my results were analyzed. Within just a couple of days the report was sent to me and I had a full debrief with Dr Hare. So, what did it show?
Independent Physiological Diagnostics (IPD) is a leading private healthcare provider specialising in home-based sleep and neurological diagnostic testing. Committed to clinical excellence, accessibility, and innovation, IPD delivers high-quality diagnostic services, including gold-standard sleep studies such as video polysomnography (PSG), all within the comfort of the patient’s home. These services support accurate diagnoses and enable tailored treatment pathways.
What my sleep cycle looked like
In the graph below, you can see how I slept for the full duration of the night, cycling between N1, N2, N3 and REM sleep, as well as periods of wakefulness.
In general, this shows excellent sleep and restorative rest.
Even though my sleep efficiency was slightly lower than usual due to an extended period of wakefulness, Dr Hare was pleased to share that I transitioned between sleep stages in a healthy way, indicating that my brain and body was functioning as it should.
You can see that after my bedtime, I very quickly transitioned through light sleep straight to deep sleep for a long time. In fact, in only took me 7.5 minutes to fall asleep (sleep latency).
Healthy sleep latency can be between 10-20 minutes, so I’m slightly below this which might indicate I was a little overtired.
Aside from a few micro disturbances, this deep sleep continued until I experienced a small amount of REM sleep before transitioning between light sleep and waking.
Below you can see a full breakdown of my sleep and how long I spend in each sleep stage in total.
I spent a lot of time awake between 2am and 3.30am. However, because I don’t usually experience these prolonged periods of wakefulness, Dr Hare put it down to the ‘first night effect’ (the anxiety of sleeping in a new environment and knowing you’re being monitored increasing your stress levels.)
My longest period of REM sleep was later on in the night during the early hours of the morning.
It’s normal for your time spent in REM to increase as the night goes on, and you can see either side of that period I was in deep sleep. During this time, I briefly transitioned between waking and N1 light sleep.
The sleep cycle broken down
So, now we know how a sleep cycle works and what it looks like, we need to understand each stage of sleep that makes up a cycle. I asked Dr Hare to help by explaining what each sleep stage means and what occurs in each one.
N1 sleep
N1 sleep is the lightest and the first stage of sleep. It can last around one to seven minutes and it’s pretty much the process of falling asleep. You’ll be able to easily wake someone if they’re in this stage of sleep.
“In young adults, stage N1 usually occupies approximately 5-10% of total sleep time. This stage is a transitional stage between wake and other sleep stages,” says Dr Hare.
“The lighter sleep stages act as ‘gatekeepers’ between wake and deep or wake and REM sleep,” she adds, which is why these stages are so essential. During N1 sleep, blood pressure, heart rate, core body temperature and muscle tone all decrease.
If you sleep through the night, you’re unlikely to spend much time in N1 as the night progresses.
N2 sleep
N2 is the second stage of sleep, and people often transition from N1 to N2 very quickly. NREM 2 sleep is deeper than N1, and we spend a lot more time (up to 50% of our sleep time) in this stage.
Our brain activity changes during this time and we see sleep spindles and K-complexes. Sleep spindles are brief, powerful bursts of neuronal firing and are thought to be essential for memory consolidation.
K-complexes are long delta waves that help to keep you sleeping, as well as also consolidating memories.
While this stage of sleep is restorative, it serves to keep us from waking up by external stimulus, allowing us to transition to N3 and REM sleep.
N3 sleep
The deepest stage of sleep is called NREM 3, and it’s sometimes referred to as ‘slow wave’ sleep. Deep sleep is essential, restorative sleep that helps the brain and the body repair itself.
We usually get around 90 minutes to two hours of deep sleep a night, however this depends on factors like your age and health.
The longest cycles of deep sleep happen early in the night, getting shorter as the night goes on and your time in REM sleep increases.
“NREM sleep and specifically N3 sleep is thought to be key in removing waste products from the brain, via the glymphatic system,” says Dr Hare.
“Impaired glymphatic clearance has been linked to neurodegenerative diseases, particularly Alzheimer’s.”
Aside from brain health, N3 also helps to maintain a healthy immune system. Dr Hare says: “Slow wave sleep (N3) is also thought to be important for immunity and, in particular for adaptive immune responses that eventually produce long-lasting immunological memory.”
This is also when most physical recovery happens. Growth hormone is released during deep sleep which helps muscles repair themselves.
REM sleep
REM sleep is characterized by rapid eye movement. “This sleep stage is unique because during REM sleep all muscles, aside from the diaphragm and the eye muscles are paralysed (the diaphragm keeps you breathing, the eye muscles create the rapid eye movements diagnostic of this sleep stage)”.
“This paralysis occurs in the context of a very active brain, in which brainwaves in fact appear similar to those seen in wakefulness,” Dr Hare adds, saying that, “this is thought to be nature’s way of preventing you from acting out your dreams.”
Aside from just dreaming, this stage is essential for “learning and for mood restoration and emotional regulation,” the sleep doctor explains.
Recent studies have found that our morning mood improves when REM sleep is intact, and worsens after a night of sleep deprivation.
“In normal adults, REM sleep increases as the night progresses and is longest in the last one-third of the sleep episode.” On average, an adult should get around 1 hour and 45 minutes to 2 hours and 15 minutes of REM sleep a night.
