Sleep metrics explained: what's your tracker trying to tell you?

We dive into deep sleep
Sleep metrics explained
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You'd be hard pushed to find a fitness tracker out there that doesn't measure sleep in some capacity, but you'd also be forgiven for not understanding what all of the data means.


We still don't have the full understanding of the function of sleep. We know we need it to live, but the exact reasons still evade science. Wearable and connected devices are making us more alert to the importance of sleep, and further improvements in technology will make these devices more accurate at tracking and helping us make use of this data, as time goes on.

Essential reading: The best sleep trackers and monitors

But not all trackers speak the same language, so we've broken down some of the terms you'll likely see cropping up when using sleep trackers so you can get a better understanding of what it all means.

Sleep duration

Sleep metrics explained: what's your tracker trying to tell you?

This is the easy one: how long did you sleep for? The optimum recommended time for sleep varies by age, but even then people differ in the amount of shut-eye they need to get to feel like they've had a good night's sleep. What we do know is that losing sleep over long periods of time leads to poor cognitive performance and other problems, such as poorer immunity functions, abnormalities with glucose and hormonal balances.

Read this: Spending some quality time with Fitbit's new sleep features

But even so, trackers often get sleep duration wrong. This is because most of them rely on movement to read how much sleep you're getting, which is a very limited tool for measuring. Some of these devices now let you edit the data afterwards, so if it says you fell asleep at 11pm but you know you didn't drift off until at least midnight, you can make that change in the morning. But as you can see, that's not the most reliable means of doing it.

Light sleep or non-REM

Sleep is all about stages and cycles. Generally, we sleep in 90 minute cycles during which we move through different stages of sleep. The first of these is often referred to as light sleep, and this is something you'll probably see on your sleep tracker.


Light sleep is a non-REM sleep (and sometimes you'll see it referred to as this) and is a transition phase to deeper REM sleep. It's easier to wake up during non-REM, and external stimuli such as noise and light can more easily interfere at this point. You also tend to move around more than you do in REM sleep; this is how many trackers determine if you're in the lighter stage.

Restless sleep

Sleep metrics explained: what's your tracker trying to tell you?

Some trackers show "restless sleep" as a metric too. This is just when you happen to move in bed, which is normal when you sleep. Restless sleep isn't a phase as such, but it's more likely to occur in light sleep than deeper sleep. If you noticed some restlessness in your tracker's results, don't be alarmed, it's normal to be restless at times during the night. However a significant amount of restlessness on a continuous basis can sometimes be a hint of sleep related problems, and if you notice you're feeling poorly rested every day then it's time to see your doctor. It's possible that a lot of restlessness could be a sign of sleep apnea if you happen to snore a lot too.

Deep sleep and Rapid eye movement (REM)

You'll sometimes see the term deep sleep being used to account for anything after light sleep, but it's also used by some devices like the ResMed S+ to describe the later stages of non-REM, where the brain starts producing delta waves.

Sometimes trackers won't differentiate between these stages and will instead record anything after light sleep as either deep sleep or "restless sleep". Some, however, will try to account for REM sleep, which is the final stage. According to Dr Jordan Stern of New York's BlueSleep clinic, the best amount of REM sleep you can get a night is between 40% and 50%, so don't start thinking you need to see a doctor when you notice you're only getting a few of those deep sleep bars a night.

More and more research is looking into the science of REM sleep. Immune function happens during this phase, and if you suppress that through lack of sleep then you're not going to synthesise or create antibodies at the same rate as someone who gets REM sleep. Memory consolidation also happens here.


"There's an understanding that REM has something to do with memory, but the exact physiological process, nobody knows," Dr Stern told us.

He also added that there is more REM sleep in the second part of the night, which is why hitting the snooze button several times in the morning is bad for you. Rapidly cycling through periods of REM and wake with light stimulating your brain isn't going to do you any favours.

Sleep hygiene

Sleep metrics explained: what's your tracker trying to tell you?

If you ever see the term sleep hygiene being used, it's not about showering before bed. It's to do with all the factors and practices that contribute to a good night's sleep.

These include getting to bed at the same time each night, not drinking very late, avoiding caffeine near bedtime, or eating a large meal before sleep.

Read next: Hugh's sleep diary – read the series

Making a positive association between your bed and sleep is also important. "The brain can definitely create associations between the bed and non-sleeping behaviours,' Dr Zeitzer, from Stanford University's Center for Sleep Sciences and Medicine, told us. "If you have a problem with sleep, this is almost always one of the first things that behavioural sleep medicine would try to change". If you regularly lie in bed to watch TV, for example, this may become a problem for some people.

Heart rate

More and more trackers have started using heart rate data to help with sleep tracking. Heart rate data helps to separate REM sleep and non-REM sleep. This is because your heart rate data is more erratic in REM sleep and lower in non-REM sleep. Some devices have started taking this HR reading and combining it with movement data to better determine which stage of sleep you're at in any given time.

It also means you can see how other things affect your sleep. Drinking a lot of alcohol before bed, for example, will often keep your heart rate high through the night – and sometimes wake you.

Got a question about sleep metrics? Let us know in the comments section.

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