LodestarDeep sleep gets a lot of attention, and a low number on your Fitbit can feel like a bad grade. Most adults get less of it than they expect, and that is usually fine.
What deep sleep is
Deep sleep is the stage sleep researchers call slow-wave sleep, or N3. Your brain waves slow into large, rolling waves, your heart rate and breathing settle to their lowest steady levels, and you are hard to wake. If someone does wake you, you feel groggy and slow for a few minutes.
It does a lot of the physical repair work of the night. Growth hormone is released mostly during deep sleep, and it seems to matter for clearing waste from the brain too. In a study in mice, the space between brain cells widened during sleep and waste was cleared much faster than when the animals were awake (Xie and colleagues, Science, 2013).
How much is normal
For a healthy young adult, deep sleep is usually somewhere around 15 to 25 percent of the night. On seven or eight hours of sleep, that works out to roughly an hour to an hour and a half. Many perfectly healthy people sit below that.
Age is the biggest factor. A large analysis of sleep studies across the lifespan found slow-wave sleep falls steadily through adulthood, while light sleep and time awake in the night go up (Ohayon and colleagues, Sleep, 2004). Someone in their sixties may get half the deep sleep they had at 25 and still be sleeping normally for their age.
Deep sleep is also front-loaded. Most of it comes in the first two or three sleep cycles, in the first half of the night. That is why a short night cuts into REM sleep more than deep sleep: your body protects deep sleep first.
Reading your Fitbit's deep sleep number
Your Fitbit splits the night into light, deep, REM and awake. It estimates these from movement and heart rate, not from brain waves, so the stage totals are estimates. Wrist-based trackers are good at telling sleep from wake, and less reliable at deciding exactly which stage you are in from minute to minute. There is more on this in how accurate Fitbit sleep tracking is.
A few practical rules:
- Watch your own average over a couple of weeks, not single nights. Deep sleep swings a lot from one night to the next.
- Don't compare your number with a friend's. Device estimates and individual differences make that comparison close to meaningless.
- A drop that lasts, alongside feeling worse during the day, is more worth noticing than a low number on its own.
What tends to raise or lower it
- Sleep pressure. The longer you have been awake and the more you were short the night before, the more deep sleep you tend to get. A late afternoon nap can take some of that pressure away.
- Alcohol. It can increase deep sleep early in the night, then fragment the second half. The trade is not a good one.
- Exercise. Regular activity is linked with a little more slow-wave sleep, especially on days you train.
- A cool, dark room and a regular bedtime help you reach and stay in the deeper stages.
- Getting enough total sleep. You can't add deep sleep on its own, but cutting the night short or breaking it up costs some.
You can't really force more deep sleep. The best lever is protecting enough total sleep at regular times, then letting your body sort out the stages. If you sleep a normal amount and still wake unrefreshed for weeks, or someone notices you stop breathing or snore heavily, talk to a doctor.
How Lodestar helps
Lodestar is an iPhone app that reads your Fitbit sleep stages through Google Health. Its Sleep score looks at how long you slept against your own sleep need, then at deep sleep, REM and efficiency against your usual range, so a lower deep sleep night is judged against you, not a chart for everyone. Tap the score to see how many points deep sleep added or took away. It also tracks sleep debt, which often matters more than any single stage.