Deep sleep: how much is enough, and can you trust your tracker's number?

What deep sleep is

Deep sleep is also called slow-wave sleep. It is one of the stages your body moves through each night. Children and teenagers need the most of it, and adults still spend a share of every night there.

How much is normal

According to the Sleep Foundation, adults typically spend 10% to 20% of the night in deep sleep. For someone who gets the recommended seven to nine hours of total sleep, that equals roughly 40 to 110 minutes.

That is a wide range, so a number near either end is not automatically a problem. The same page suggests paying attention to how you feel in the morning and through the day, and talking to your doctor about how much sleep you really need for your age and overall health.

Why it falls as you get older

Children and adolescents need the most deep sleep. After that, the Sleep Foundation says, deep sleep starts to decrease over time and into older age. Reduced deep sleep is a normal part of aging, with a gradual decline that levels off in your 70s.

So if a parent or grandparent shows fewer deep sleep minutes than you do, that alone is not a warning sign. Compare a person with their own earlier nights, not with someone twenty years younger.

How to give your body a better chance

The Sleep Foundation's sleep hygiene advice is plain and free. Nothing here guarantees more deep sleep, but these are the habits it recommends for better sleep overall:

• wake up at the same time every day, weekends included
• set aside about 30 minutes to wind down, and keep a 30 to 60 minute screen-free gap before bed
• get daylight, especially sunlight, and stay physically active
• avoid caffeine, alcohol and smoking later in the day, and skip heavy late meals
• keep the bedroom cool (the page suggests around 65 degrees Fahrenheit, about 18 degrees Celsius), dark and quiet

How far to trust a tracker's deep sleep number

Not completely. The Sleep Foundation says sleep trackers can provide meaningful data but are not perfectly accurate, and that the real test is a comparison with a home sleep test or an overnight sleep study. A 2022 study in the journal Sensors compared six popular wearables, including a ring, with that lab standard. The devices agreed well on sleep versus awake (86% to 89% agreement), but only moderately on specific stages: multi-stage accuracy ran from 50% to 65%. The authors concluded that all six need further improvement for assessing specific sleep stages.

That means a tracker saying "1 hour 12 minutes of deep sleep" is a best guess. Use it for patterns across weeks, such as a run of poor nights after you change something, not as a score to chase. The Zenlyf Ring is worn through the night for this kind of trend, and it has no published sleep-stage validation yet. For more on devices, read ring or smartwatch for sleep tracking.

When to see a doctor

If you are tired every day even after enough hours in bed, speak to a doctor. A tracker cannot diagnose a sleep problem.

Common questions

Keep reading

• Does a smart ring track sleep better than a watch?
• What is heart rate variability, and should you track it?
• Pulse oximeters and blood oxygen: what is a normal oxygen level?
• How to choose a sleep tracker, and what to expect from one