What is deep sleep? | Tom’s Guide
It’s common to describe getting a good night’s rest by saying we’ve had a really deep sleep. But in reality, actual deep sleep is just a portion of the 7 to 9 hours sleep that’s recommended for adults.
Deep sleep — also known as slow wave sleep or N3 sleep — is still vital though, as it’s “the most restorative stage of sleep,” explains UKCP-registered psychotherapist specialising in sleep and author of How to Be Awake (So You Can Sleep Through the Night), Heather Darwall-Smith.
It’s when, she says, “your body focuses on physical repair and recovery,” adding that during it, “your breathing and heart rate slow, your muscles relax, and your brain waves shift into slow, steady rhythms.”
This revitalizing sleep stage is involved in so much, from our energy levels to cell regeneration, repair of tissue and bones, memory and having a strong immune system.
So, for Sleep Awareness Week 2025, we’ve spoken to a variety of sleep experts and explored the research on this sleep stage to answer everything there is to know about deep sleep. What it does and how much we need, what happens if we don’t get enough, and more…
What is deep sleep?
“Deep sleep is also known as slow-wave sleep and is the third and deepest stage of non-REM sleep,” says Dr. William Lu, Dreem Health Medical Director and sleep medicine physician.
“[It] occurs during the first half of the night and is characterized by slow brain waves, reduced muscle activity, slowed breathing and heart rate, and diminished consciousness,” he adds.
Dr Lindsay Browning, a sleep expert and chartered psychologist at Trouble Sleeping, explains a little more about why deep sleep is also called slow wave sleep. “During deep sleep, neurons in our brain pulse in slow waves,” she says.
According to research, deep sleep is also the hardest stage to be woken from, even if there are loud noises around you.
“During this part of sleep, your brain is very far from consciousness and you are hardest to wake up (compared to light and REM sleep),” Dr Browning adds.
Added to that, a 2019 review of research into the causes of sleep inertia (typically grogginess and decreased cognitive ability that occurs for around 30 minutes after waking) found that you’re more likely to experience it if you’re woken up during deeper sleep.
How much deep sleep do we need?
Like other sleep stages, including REM sleep, the amount of deep sleep we get generally decreases as we age.
“Children and younger adults tend to have more deep sleep than older adults,” says Dr. Browning.
Generally, we’ll have between 1.5 to 2 hours in deep sleep each night
It makes sense when you think about the function of deep sleep in growth and repair of muscles and bones, which is naturally happening more for growing young people.
By the time we are adults, though, we are likely to spend around 13 to 23% of our entire sleep in deep sleep, according to studies.
That means that, generally, we’ll have between 1.5 to 2 hours in deep sleep each night, though these figures depend on the length of your overall sleep.
Interestingly, researchers looking at changes in sleep characteristics as we age reported a decrease in the percentage of deep sleep as adults aged, up to 60 years old.
When does deep sleep occur?
“Light and deep sleep are collectively known as non-REM sleep,” Dr Browning explains.
During the course of the night, you’ll typically go through four to six sleep cycles, with research showing that each cycle can last between 90 to 110 minutes.
Non-REM sleep accounts for the first three of the four stages of each sleep cycle, and deep sleep, or N3, comes after N1 sleep (when we’re transitioning from wakefulness to sleep) and N2 sleep (the lightest sleep stage).
Deep sleep is the third and final stage of non-REM sleep before our body transitions into REM sleep (which is the phase we experience our most vivid dreaming).
Oxford Health notes that we generally spend more time in the deep sleep stage during the first part of the night.
What does deep sleep do?
While its the combined effect of the four sleep stages working together that maintain our overall health and wellbeing, there are specific ways deep sleep in particular helps us.
1. Growth and repair of tissues and bones
It’s incredible to think that while we’re in deep sleep, our bodies are actively healing thanks to the hormones that are released during this sleep stage.
“Deep sleep significantly contributes to growth and repair by triggering the release of human growth hormone from the pituitary gland, which stimulates the body to rebuild and repair tissues like muscles and bones,” says Dr Lu.
And, if you were wondering why we typically need much more sleep when we’re younger, a need for more deep sleep is part of the reason.
“This is why kids and teenagers need extra sleep – it’s when they grow,” explains Darwall-Smith.
A 2024 review of studies into the relationship between sleep, growth hormone (GH) and growth hormone deficiency (GHD) in children noted that, “during deep sleep, the body produces and releases the most GH. In children affected by GHD, the production of this hormone may be reduced or absent.”
2. Strengthens the immune system
If you’re fighting a cold, or simply trying to stay healthy, deep sleep is your friend. Research shows this sleep stage is essential for keeping the immune system strong, and experts explain that this is thanks to the role it plays in the generation of vital cells and proteins.
What are cytokins?
Cytokines are signalling proteins whose role in your body is to help prevent inflammation.
“Deep sleep strengthens the immune system by facilitating the production of crucial immune cells like T-Cells, regulating the release of cytokines and enhancing the body’s ability to recognize and respond to invading pathogens,” explains Dr Lu.
Darwall-Smith agrees, saying that “deep sleep helps the body produce infection-fighting cells and improves how it responds to vaccines.”
“You’re more likely to get sick and stay sick longer without enough,” she adds.
3. Cell regeneration
Those growth hormones that are released in deep sleep are also involved in regenerating the body’s cells, helping to keep you functioning well.
“Deep sleep contributes to cell regeneration by facilitating the release of growth hormones, which activity promotes tissue repair and cell renewal through the body,” Dr Lu says.
Think of it as “hitting the refresh button overnight,” Darwall-Smith says.
“Your body clears out damaged or dead cells and creates new ones, keeping your organs and tissues working properly,” she adds.
4. Increases blood supply to the muscles
If you’re training for a marathon, had a big session at the gym, or simply been particularly active during the day, deep sleep is going to help to make sure your muscles get the oxygen and nutrient-rich blood they need.
“Deep sleep increases blood supply to the muscles… allowing for better oxygen and nutrients to the muscle tissue during this restorative sleep stage,” Dr Lu tells us.
This means they’ll repair themselves and you’ll wake up with less aches and pains.
5. Restores energy
Ever woken up from a too short or disrupted sleep feeling groggy and exhausted? Darwall-Smith explains that this is because without getting enough deep sleep, “you’ll feel sluggish no matter how long you’ve been in bed.”
All the restorative work that is taking place in your body and brain during deep sleep (as we discussed above) helps us to feel bright and alert the next day.
“Deep sleep is the primary stage where your body repairs and renews itself, leading to a significant restoration of energy,” Dr Lu says.
And, according to Harvard Health, this sleep stage seems to be the most significant when it comes to our energy levels because it boosts our ability to make our body’s energy molecule, Adenosine triphosphate (ATP).
6. Helps with learning and storing memories
Deep sleep plays an important role in learning and memory storage, “by facilitating a process called ‘memory consolidation,’ where the brain actively strengthens and stabilizes newly acquired information, transferring it from short-term to long-term storage,” Dr Lu explains.
One study showed that deep (slow wave) sleep helps to maintain the brain’s learning efficiency, while there is plenty of research that looks at the relationship between deep sleep and memory processing.
Deep sleep is when your brain organises and stores information from the day, locking in what you’ve learned
One study even found that as deep sleep declines as we age, so does this ability to consolidate memories.
If you’ve ever sacrificed sleep and found that you’ve struggled to work through tasks the next day, or found that you’ve forgotten a normally familiar word, it could be a result of not getting enough deep sleep.
“Deep sleep is when your brain organises and stores information from the day, locking in what you’ve learned. Without it, studying, problem-solving, and recalling basic facts get harder,” Darwall-Smith adds.
What happens if you don’t get enough deep sleep?
“If you’re missing out on deep sleep, you might feel more than just tired,” says Darwall-Smith. She explains that there are multiple ways a lack of deep sleep and sleep deprivation can impact both your body and brain.
Among those are physical exhaustion and slower healing, as deep sleep is when the body repairs itself. Without it, “muscle soreness lingers longer, and injuries take more time to heal,” she says.
Since deep sleep is important for helping the immune system to build up defences against infections, “Less deep sleep = more colds, flu, and long-term health risks,” she explains.
Ever woken up with a feeling of brain fog, or like we mentioned above, had trouble with your memory during the day after a bad sleep?
Long-term deep sleep deprivation has been linked to higher blood pressure, heart disease, and type 2 diabetes
“Deep sleep matters for memory formation. Without it, you may feel mentally sluggish, struggle to focus, and find it harder to retain new information,” Darwall-Smith says.
There’s even the possibility of an increased risk of chronic conditions. “Long-term deep sleep deprivation has been linked to higher blood pressure, heart disease, and type 2 diabetes because the body isn’t getting its proper recovery time,” she explains.
Dr Lu adds that “insufficient deep sleep can lead to an increased risk of chronic diseases, impaired cognitive function, hormonal imbalances, and certain mental health disorders,” adding that you may also “suffer from difficulty learning, concentrating and remembering and mood swings.”
How to get more deep sleep?
Keep a regular sleep schedule
One of the simplest ways to get more deep sleep is sticking to a consistent sleep schedule.
It’s as straightforward as going to bed and setting your alarm at the same time each night and morning. This will help to regulate your sleep cycle.
The benefit of this is that your body clock will align with your sleep/wake schedule and you will be releasing melatonin (the hormone that helps us sleep) at the right time each evening.
You’ll (hopefully!) begin to notice that you’re falling asleep with more ease and having less disrupted sleep, meaning that you’ll be able to cycle effectively through the four stages of sleep and get the amount of deep sleep you need.
Your energy levels throughout the day should also be more regulated, meaning you’re less likely to experience dips and feel sleepy.
Avoid caffeine and alcohol
An afternoon ‘pick-me-up’ coffee might seem like a good idea if you’re struggling with your energy levels during the day, but it’s not necessarily great for sleep.
“Caffeine can reduce the amount of deep sleep that someone gets, so it may be wise avoiding caffeine close to bedtime,” says Dr Browning.
Similarly, alcohol might give you the illusion that you’re getting a good sleep because you may drop off better after a few glasses of wine at night. However, it can actually disrupt our sleep cycles.
Research has shown that dependence on alcohol can actually decrease the amount of deep sleep we get, while another small study showed that even small amounts of alcohol reduced slow wave (deep) sleep in participants.
Experts say that drinking only modest amounts and at least 3 to 4 hours before you plan to go to sleep is the best way to avoid any of the negative affects of booze.
Be active during the day
Making sure you’re not sitting at your desk, or on the couch, all day is going to have a positive impact on your ZZZs.
Dr. Browning previously explained to us that “the more you exercise, the more deep sleep you will have. Deep sleep helps you to feel refreshed when you wake up and helps with sleep continuity.”
Exercise during the day can help to regulate your sleep cycle and even boost melatonin production (yep, it’s that sleepy hormone again), meaning you might find it easier to fall asleep and have an undisturbed night’s rest.
While there have been multiple studies looking at the best time to exercise to help with sleep (and one study in 2016 showed that adults who exercised regularly found they had more deep sleep after exercising 1.5 hours before bedtime,) generally its believed that if you take part in intensive exercise right before bedtime you can stimulate your nervous system, which may not be conducive to drifting off.
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