If you’ve ever woken up from a full night’s rest feeling unrefreshed — like you hardly slept at all — then you understand the difference between getting enough sleep and getting enough good sleep. When it comes to the mental and physical benefits of sleep, both quantity and quality matter.

Sleep quality is a multifaceted concept; researchers might look at several aspects of rest in order to assess it. The most crucial aspect, though, is spending enough time in deep sleep, also called slow-wave sleep. 

Here’s a guide to this critical sleep stage — what makes sleep “deep”, why it’s so important, and what you can do to increase it and optimize its benefits. 

What are the different sleep stages, and which one is deep sleep? 

Scientists first categorized sleep into stages in the 1930s. Using EEG technology, they observed that brain waves — the visual representation of electrical brain activity — changed over the course of sleep in predictable ways. The classification of sleep stages has evolved over time. Among other things, experts now recognize four stages in the human sleep cycle instead of five. The commonly recognized sleep stages are:

Stage 1 (N1)	Light non-REM (non-rapid eye movement) sleep
Stage 2 (N2)	Deeper non-REM sleep
Stage 3 (N3)	Deepest non-REM sleep
Stage 4 (REM)	REM sleep

 

One cycle takes 90 to 120 minutes, and most people log four to six sleep cycles per night. (Related: Learn more about the sleep stages in our guide to understanding your sleep cycle.) 

If you feel like you’re tossing and turning more in the second half of the night, that’s normal. Deep sleep is not distributed evenly across the night: Most deep sleep occurs in the first half, with the longest deep-sleep phase typically happening during the first sleep cycle. Then, in the second half of the night, REM phases take up a larger proportion of each sleep cycle, and deep sleep periods become much shorter. 

How much deep sleep do adults get per night? 

On average, adults spend up to 20 percent of their night’s rest in deep sleep. So, if you get eight hours of sleep in total, you’ll likely log somewhere between 60 and 110 minutes of deep sleep. But age plays a role, as deep sleep decreases over time.

The relative amount of deep sleep peaks during early childhood, drops significantly after puberty, and then continues to decline somewhat gradually. While older adults get the least deep sleep, the rate of decline is fastest between early adulthood and middle age. Then it slows down at around 45 years old. Deep sleep comprises:

• 25 to 35 percent of nighttime sleep for kids ages seven to 12
• 15 to 20 percent for younger and middle-aged adults
• Less than 15 percent for older adults (60 and older)

Multiple factors likely contribute to the later-in-life drop-off in deep sleep. These include changes in brain chemicals that help regulate circadian rhythms, as well as changes in sleep-related hormones such as growth hormone and cortisol. But experts are still figuring out exactly why people get so much less deep sleep starting around 60 or 65, and whether they actually need less than younger people.

One simple explanation is that older adults get less deep sleep because, physiologically, they don’t need as much in order to feel refreshed and reap its restorative benefits. In this view, deep-sleep reduction is just a normal age-related change and neither a symptom nor a cause of health problems. There’s some support for this idea — in studies, children and younger adults have reported more severe next-day sleepiness from reduced deep sleep, compared to older adults. But another theory is that reduced deep sleep in advanced age contributes to age-related cognitive impairments, and that older adults would benefit from getting more of it. 

Why is deep sleep so important?

Deep sleep is the most restorative sleep stage, with the strongest link to sleep quality. In fact, it’s the only sleep stage that’s individually associated with subjective reports of waking up feeling well-rested. Research has linked the restorative benefits of deep sleep to various positive outcomes, such as stronger athletic performance and prosocial behavior like charitable giving and kindness toward strangers. (Experts don’t have a solid hypothesis for explaining the latter one yet.)

Importantly, deep sleep has numerous functions that are vital to cognitive functioning and physical well-being. Here are a few: 

Memory support: Deep sleep enhances memory consolidation, the process of converting newly acquired information into long-term memories. More specifically, deep sleep is important for declarative memory, which involves consciously recalled facts and can be further broken down into personally relevant details (like your childhood best friend’s phone number or the date you adopted your dog) and general knowledge (such as grammar rules, constitutional amendments, or anything that might come up on Jeopardy!). 

The neural basis of deep sleep’s memory improvements seem to be rooted in hippocampal activity. Certain areas of the hippocampus are activated when you learn new information, and those same areas are later reactivated during deep sleep. So the brain seems to “replay” freshly formed memories to turn them into permanent ones. Research has found that higher hippocampal activity during deep sleep on the night after you learn new information is linked to stronger next-day recall. (But keep in mind, REM sleep is also vital for memory processing, so deep sleep isn’t the only player here.)

Mental energy conservation: During the day, your brain absorbs a constant stream of information. Processing and making sense of everything requires many synapses to be highly active. This becomes unsustainable after a while — it depletes brain resources and leaves synapses too “saturated” to allow for more learning. During sleep, synapses in the brain are selectively weakened to help counteract the effects of daytime cognitive demands. This process is called synaptic downscaling.

Hormone release: The secretion of growth hormone (GH) significantly increases during deep sleep, especially the first deep-sleep phase of the night. This hormonal surge is essential for the regeneration and repair of bodily tissue and muscle growth in adulthood, and for regulating metabolism in childhood and adolescence. 

Waste clearance: The glymphatic system is a recently discovered network of tubes that pumps cerebrospinal fluid through brain tissue to remove junk. Research suggests the glymphatic system is most active during deep sleep, when it performs the critical task of clearing out toxic proteins associated with dementia.

Immunity protection: Research suggests that slow-wave sleep promotes adaptive immunity, where the body builds up its defenses against viruses and other types of pathogens. One study showed that slow-wave sleep following vaccination boosted immune responses. 

What happens when you don’t get enough deep sleep?

Inadequate deep sleep can compromise memory recall and inhibit the other benefits of sleep mentioned above. Over the long term, deep sleep deficits have been linked to Alzheimer’s disease, among other types of dementia, in multiple ways. Deep sleep changes often begin before dementia symptoms emerge. Compared to healthy controls, people with mild cognitive impairment get less deep sleep and Alzheimer’s patients get significantly less.

Other health problems associated with inadequate or decreased deep sleep include depression, schizophrenia, and PTSD; weakened immunity; reduced insulin sensitivity, which is linked to Type 2 diabetes; and increased blood pressure.

What are some reasons for not getting deep sleep?

An array of health issues can lead to reduced deep sleep, including:

• Sleep disorders, such as insomnia and sleep apnea
• Stress
• Chronic inflammation
• Pain conditions like arthritis 

Substances including alcohol and opioid painkillers can also disrupt deep sleep. 

How is deep sleep measured?

A few different tools can measure how much time you spend in each sleep stage, including deep sleep. The gold standard tool is EEG (electroencephalography), which records the frequency and amplitude of electrical activity in the brain, traditionally via electrodes placed on the scalp. EEG has been utilized since the 1930s. Today, it might be used on its own or as a component of polysomnography, a comprehensive sleep evaluation seen in clinical sleep studies. This is the core technology of Tone Buds, which studies indicate show measure deep sleep (and other sleep stages) with accuracy comparable to clinical EEG.

Consumer sleep-tracking devices and phone apps tend not to be quite as reliable as EEG (or polysomnography) because they don’t directly measure brain activity. Instead, they infer which stage of sleep you’re in based on proxy metrics. For example, they may measure bodily movement — a method called actigraphy — to help determine when you’re awake versus asleep.   

Multi-sensor devices that monitor breathing and heart rate generally work better than those that rely on actigraphy alone. But research suggests it’s still common for these trackers to mix up sleep stages. In one study that evaluated seven different sleep trackers, one wearable product, the Fitbit Alta HR, stood out as the most accurate at identifying REM and light sleep. (The study also included products from Garmin, Fatigue Science, EarlySense, ResMed and SleepScore.) None of the devices, however, performed that well when it came to deep sleep. All of them misclassified it as light sleep at least some of the time.

What can you do to increase deep sleep or optimize its benefits?

It might seem hard to target a specific stage of sleep, but researchers have discovered ways to increase slow-wave brain activity, as well as improve subjective sleep quality (an indicator of sleep depth).

1. Eat a Mediterranean diet and get regular movement.

Plenty of research shows that regular exercise and a balanced diet are generally helpful for sleep. Mediterranean-style diets — which are heavy on fresh fruits and vegetables, healthy fats like olive oil, beans, legumes, whole grains, and seafood — have been linked to higher-quality sleep. And research suggests that multiple types of exercise can enhance slow-wave sleep in adults of all ages. One study found that moderate-intensity exercise in the afternoon led to the largest increases in deep sleep. Both high-and-low-intensity exercise were also linked to deep sleep, but the impacts were smaller.  

2. Reduce time in bed. 

Getting less sleep overall doesn’t necessarily mean getting less deep sleep. A technique called sleep restriction involves reducing how much time you spend in bed at night, with the goal of ensuring the majority of time spent in bed is time spent asleep. In experiments, people who restrict their time in bed to five hours typically spend less time than usual in Stage 1, Stage 2, and REM sleep, but more time in deep sleep. In practice, this means going to bed when you’re drowsy and ready to fall asleep, and not spending hours in bed procrastinating or doing other tasks.

3. Use audio to tap into deep-sleep benefits.

Exposure to pink noise can help slow brain waves to induce deep sleep and increase the amount of time spent in deep sleep. This type of sound, which consists of lower frequencies and can feel more gentle than white noise, helps improve deep sleep by gently synchronizing brain activity with the slow waves that naturally occur during deep sleep.

Research has also shown that a technique called acoustic stimulation (AS) can lead to increased slow-wave activity as well as overnight memory improvements. AS entails delivering precisely timed sounds during deep sleep to help improve the recall of information learned during the day. Exposure to pink noise can also help slow brain waves to induce deep sleep and increase the amount of time spent in deep sleep.

4. Avoid waking up during deep sleep.

Compared to other sleep stages, waking up during deep sleep causes worse sleep inertia, a state characterized by disorientation, mental fogginess, and temporary deficits in cognitive performance. Studies have shown more significant disruptions in functional brain connectivity when people wake up during deep sleep versus Stage 2 sleep. Using a smart alarm that accounts for sleep stage can help you avoid deep sleep disruptions and wake feeling more alert. Similarly, limiting naps to 10-20 minutes reduces the chance of waking up groggy. 

The Tone Take: Deep sleep is one of many power players in your body’s sleep and recovery lineup

We now know a lot about the benefits and defining features of different sleep stages, and deep sleep stands out as being particularly important for rest, recovery, and memory. Still, many of the crucial processes that occur during sleep are dependent on multiple stages. Take memory consolidation: Deep sleep deserves the most credit for the library of factual knowledge seared into your neocortex, but numerous studies also show that sleep spindles in Stage 2 help facilitate memory consolidation. So, think of deep sleep as a critical part of an interdependent team, rather than a solo act.