Deep sleep, also called slow wave sleep (SWS) is one of the four sleep stages we experience each night, along with wake, light sleep, and REM sleep. Deep sleep is the stage where your body heals itself, regenerates cells and tissue, and strengthens your immune system.
It’s called deep sleep because it’s the most restful stage of sleep and the most difficult to wake directly from. Sleep experts call it slow wave sleep because the brain waves slow down significantly. Your heart rate and respiratory rate also decrease during deep sleep as your body recovers for the next day.
Adults generally average 1-2 hours of deep sleep per night, somewhere between 15 and 25% of your nightly sleep. Most of our deep sleep comes in the first sleep cycle of the night, usually 45-90 minutes. Age has a major effect on how much deep sleep we get each night and how much we need.
Children generally get more deep sleep than adults and experience most of their growth during this stage. Teenagers also need more slow wave sleep than adults for the same reason. As we age, the time we spend in deep sleep decreases at a rate of about 1.7% per decade.
There may be several reasons for these declines such as the aches and pains that come with aging and a greater likelihood of daytime napping. For the most part, men experience steeper declines of deep sleep. Men aged 70 and older spend an average of 5% of their time asleep in deep sleep, while women in the same age bracket spend between 15% and 20% of theirs in deep sleep. This may be because older women tend to go to bed earlier than older men.
There are a number of reasons you may be losing sleep at night, such as napping or drinking coffee or tea too late in the day. Sleep disorders like sleep apnea can disrupt your deep sleep leaving you with symptoms of sleep deprivation. Symptoms of deep sleep deprivation can include:
Worsening chronic pain: People who suffer from chronic pain may experience worsening symptoms when they do not get enough deep sleep. This can become a cycle in which chronic pain leads to poor sleep leading to worsening pain.
Feeling exhausted: When you wake up not just tired but feeling a lingering fatigue that is consistent and limits your ability or desire to do anything, it may be due to a lack of deep sleep.
Difficulty remembering: Your brain processes and consolidates new information during SWS and missing deep sleep can negatively affect your memory. Lost deep sleep can also mean you’re less alert and focused the next day and less able to learn things.
Brain fog: If you’ve gotten your 90 minutes of deep sleep at night you should wake feeling alert and refreshed. Not getting enough SWS can lead to poor concentration, an inability to focus, decreased mental clarity, and put you in a haze that prevents you from staying focused when you need to be.
Other symptoms include:
LEARN MORE: How are Deep Sleep and REM Sleep Different?
The easiest way to get more SWS at night is to avoid napping during the day. It’s natural to feel tired in the midafternoon, but don’t stretch out for a catnap. Being awake for extended periods can enhance your “homeostatic sleep drive,” which means that the longer you stay awake the more you want to sleep. When you finally go to bed, you’ll likely increase your deep sleep.
Maintaining sleep consistency, or going to bed and waking up at the same time every day can help support deep sleep.
Many people think that a drink in the evening helps them sleep better, but alcohol has a significant negative effect on the quality of sleep you’re getting. Your body cannot reach restorative deep sleep while it’s trying to process alcohol. You’ll end up getting more light sleep and less of the restful deep sleep you need, and you will not feel refreshed upon waking. If you’re trying to get more slow wave sleep, avoid the evening beer or cocktail.
READ MORE: How to Get More Deep Sleep
The WHOOP Sleep Coach can help you get more sleep by providing bedtime recommendations to encourage you to go to bed at the same time each night. The haptic alarm wakes you when you’ve reached your sleep or recovery goals, or at the exact time you need to wake up.
The WHOOP app also shows you the benefits of making positive changes in your sleep schedule. For example, if you start avoiding those afternoon naps you may see an increase in your deep sleep compared to the previous 30 days.