How to track sleep on a smartwatch or fitness tracker

Many of us routinely skimp on sleep – in fact, two thirds of adults throughout developed nations get less than eight hours' a night. 

Getting the NHS-recommended seven to nine – and making sure it’s good quality sleep – can improve your health, memory, mood, appearance and decision-making. Insufficient sleep has health impacts such as affecting your immunity and increasing your risk of health problems, as well as affecting your weight.

A fitness tracker or smartwatch with sleep tracking can offer valuable data to help you optimise your bedtime routine. Discover how this data can help with sleep – and the limitations to be aware of. 

Our smartwatch reviews and fitness tracker reviews reveal the sleep tracking features on offer in each model. 

Video: How to improve your sleep with a fitness tracker or smartwatch

What can sleep-tracking wearables monitor?

Many fitness trackers and smartwatches, even the cheap ones, offer some level of sleep tracking. Typical stats they monitor include:

• Sleep duration – how long you've been asleep or awake. 
• Sleep quality data – whether your sleep has been restless or good quality. 
• Sleep stage data – how long you spent in each stage of sleep (Light, Deep, REM). 
• Sleep cycle alarm – to wake you up at the optimum point in your sleep cycle.
• Sleep problem detection – some devices claim to detect sleep apnoea, a potentially serious health issue in which breathing stops and starts while you're asleep. Any flagged abnormalities should be discussed with your doctor. 
• Sleep tips – many will give you tips for sleeping better and allow you to record factors that might be affecting your sleep, such as exercise and caffeine intake. This can be helpful for understanding what's impacting your sleep patterns.

Do sleep-tracking wearables really work?

To an extent, yes, though they aren't perfect. Kathryn Pinkham, sleep expert and founder of The Insomnia Clinic, told us 'Sleep trackers can offer useful insights for those who are interested in metrics like how much time you spend in bed or how much time is spent asleep, and for someone who doesn't prioritise sleep, the data could highlight you are not getting enough sleep, prompting the move to making sleep a higher priority.'

Kathryn recommends the following to help interpret sleep data in a meaningful way:

• Use data sparingly: Review numbers once a week to spot trends, not every day.
• Focus on how you feel, not the score: The metric may not be accurate enough to change your day based on it. We can cope with more than we think even when tired, so try to focus on how you feel and not the score you get. 
• Complement with Cognitive Behavioral Therapy for Insomnia (CBT-I) strategies: If you're struggling to sleep, don’t change your routine based on data; work on resetting your body clock, strengthening your sleep drive, re-setting your mindset towards sleep (rather than to over focus on it) and reducing vigilance. These are the factors that really impact our sleep quality. CBT for insomnia is an evidence-based treatment which can guide you, step by step how do make these changes.

Older and cheaper wrist-worn sleep trackers rely on an accelerometer (which tracks movement) to auto-detect sleep. If you're inactive for a certain length of time, or your movements – such as rolling over – are considered to be typical sleep behaviour, then the tracker will assume you're sleeping. 

Newer models that can detect your heart rate and breathing are generally more accurate, as they have more data to go on. However, even these can be fooled. Experts agree that consumer-grade trackers have significant limitations.

• They overestimate sleep: According to Professor Guy Leschziner, consultant neurologist at Guy's and St Thomas' Hospitals Sleep Disorders Centre, devices are 'less accurate at picking up short periods of wake in the middle of the night, and so may overestimate sleep as a result'. Trackers can be misled when you're lying still but awake, as the device will often incorrectly show that you are asleep.
• Sleep stage data is an estimate: Tracking your 'deep' or 'REM' sleep is not scientifically precise. Pinkham confirms that while devices provide this data, 'no consumer device matches medical-grade accuracy (polysomnography)', which involves measuring brain activity.

Read our guide to better sleep for more tips from sleep experts. 

Are sleep trackers helpful for insomnia and other sleep disorders?

The consensus from the experts we've spoken to is a clear 'no'. And for those with consistent sleep issues, trackers can do more harm than good. Prof. Leischziner puts it bluntly: 'if you have insomnia, a sleep tracker should be avoided'. Kathryn Pinkham also recommends clients and course users stop tracking their sleep entirely when working to resolve insomnia.

There are various reasons why. One is the risk of sleep anxiety, fuelled by the addition of extra data to consider. If you already know you've had a bad night, there's a risk that having a tracker confirm that only adds pressure. This can lead to a state of hypervigilance about sleep, which is one of the key factors that keeps insomnia going. This obsession with achieving perfect sleep scores has been termed 'orthosomnia'.

In addition, sleep trackers are not a substitute for medical advice. Pinkham states they 'are not diagnostic tools – they cannot replace medical testing such as polysomnography. Prof. Leschziner warns that the accuracy of trackers drops in patients with sleep disorders, and the data 'really do need to be interpreted with caution'.

Ultimately, if you are concerned about your sleep, the advice is to look beyond consumer technology. Most trackers will only offer very basic sleep hygiene advice as a treatment option, but these are unlikely to help someone with a consistent sleep issue. 

As Leschziner puts it, 'If you think you may have a sleep disorder, the first port of call should be your GP, not the gadget shop'.

What does a typical sleep-cycle look like?

A typical night's sleep consists of several cycles of deeper and lighter sleep stages, known as REM (rapid eye movement) and Non-REM sleep.

We typically cycle through Non-REM to REM stages every 90-110 minutes, with REM cycles getting longer each time.

Non-REM sleep

• Stage 1 – lighter sleep, where you drift in and out of sleep and can be easily woken. Your eyes move slowly and muscle activity slows.
• Stage 2 – while sleep is still fairly light, your body prepares for deep sleep. Eye movements, heart rate and brain waves slow down, and your temperature drops. 
• Stages 3-4 – known as deep sleep or slow wave sleep, your heart rate and breathing rate are at their lowest, muscles are relaxed and brain waves slow down further. In this stage, you'll be difficult to rouse and can feel disorientated if woken up. 

REM sleep

Vivid, hallucinogenic dreams that seem to form a narrative come from REM sleep. Breathing becomes faster, irregular and shallow and your brain returns to a more wakeful state. Your limbs may become paralysed to stop you acting out your dreams. 

Insufficient REM sleep can blunt our ability to discern others' facial expressions, making it harder to navigate social situations. Lack of REM also affects memory and general cognitive performance. 

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Sleep tracking on an Apple watch

If you're using watchOS 7 or later (compatible with all Apple Watches apart from the Series 1 and 2), you can use the Sleep app on your Apple Watch to track sleep, including:

• set up bedtime schedules and goals
• set a wake-up alarm
• set a wind-down time, meaning the watch will go into Sleep Mode (which turns on Do Not Disturb) for a period before the sleep time you're aiming for
• find out how much sleep you had
• view your sleep trends over the past 14 days.

We found that older Apple watches (pre-Series 8) struggle with enough battery life to get through the night for sleep tracking. Head to our guide on the best Apple Watch to find the right watch for your needs. best match for your needs. 

Sleep tracking with a Fitbit

All Fitbit fitness trackers and smartwatches will track your sleep when you wear them to bed. After an hour of your body being immobile, your Fitbit will detect that you're asleep. You'll get information on:

• how many hours you've slept for and your weekly average
• your sleep score for the week and your weekly average
• your sleeping and waking up times for the week.

Fitbits with heart-rate monitors (apart from the Fitbit Charge HR and Fitbit Surge) are designed to track sleep stages and to give you graphs showing daily, weekly and monthly stats. Some also measure blood oxygen levels (pulse oximetry).

There's also the option to subscribe to Fitbit Premium with certain devices to get a more detailed 'sleep profile' and snoring monitoring. 

See our Fitbit smartwatch reviews and head to our guide on the best Fitbits to see which ones we recommend.

Sleep-tracking rings

If you don't fancy a wrist-worn wearable, another option is to buy a smart ring. This is still a relatively niche market, but it is growing.

Many smart rings focus on smart payments, but some also have sleep-tracking capabilities, including the Oura (currently only available from the Oura website), which has a special focus on sleep and health.

Smart rings collect data such as your movement, heart rate, heart rate variability, body temperature and breathing to give you insights about your health and habits.

Read our guide to the best smart rings for more.

Smartphone apps for sleep-tracking

If you don't want to wear sleep tech to bed (or can't, because the short battery life requires you to stick it on charge every night) there are plenty of smartphone apps you can use instead. 

Smartphone apps rely on an accelerometer to detect movement and, by extension, sleep. So, even more than with wrist-worn devices, you can't expect them to track super accurately. 

Still, many offer useful tools – for example, playing 'dreamscapes' (music, voiceovers and sound effects) to help you sleep better, giving you tips from sleep experts and teaching you cognitive techniques and behavioural strategies to reset your sleeping patterns. 

The NHS recommends the following smartphone apps:

• Pzizz – free for certain features, with the option to make in-app purchases
• Sleepio – free with GP referral
• Sleepstation – free with GP referral.

Looking for a more general mindfulness and meditation app? See our guide to mindfulness apps compared

Which? tests of sleep-tracking wearables

When we test wearables with sleep-tracking functionality our testers wear them for up to three nights, assessing:

• how detailed and easy to understand the sleep data is
• how plausible the sleep durations seem
• any outstanding capabilities or unusual features. 

If the sleep functionality is hard to use, or the data is hard to make head or tail of, we'll flag this in our reviews. 

If you do buy a wearable for sleep-tracking, make sure you pick one that's light and comfortable enough to keep on your wrist all night. 

Fitness trackers tend to be smaller and more comfortable to wear than smartwatches, so might be a better bet if you're particularly keen to use this function.

Use our fitness tracker reviews and smartwatch reviews to find a model that ticks your boxes.

How to sleep better

If you're struggling to drift off despite avoiding obvious things like avoiding caffeine, phone screens and heavy meals before bed, here are some other tips to try:

• Stick to regular sleeping hours, rather than 'catching up on sleep', even if you've had a bad night's sleep the night before.
• Consider an eye mask to block out light – our eye mask reviews reveal the best models for a good night's sleep. 
• Don't believe the myth that older people need less sleep. You may struggle to get as much restorative sleep as you age, due to medications, medical conditions and day-time dozing. But you should still aim for seven to nine hours at night.
• Cut down on evening alcohol. Alcohol makes you feel drowsy but increases your chances of waking early and blocks REM (leaving you feeling groggy next day). As it's a diuretic, it also makes it more likely you'll wake up needing the bathroom.
• Understand your rhythm. There are biological and evolutionary reasons why some of us are owls and some are larks. If you're an owl forced into an early-to-bed routine, don't beat yourself up if you can't sleep instantly. Get sunlight first thing – eat your breakfast near a window – to help reset your circadian clock
• Keep your bedroom dark and cool (18-24°C). Use our mattress reviews to buy a comfortable mattress, and don't neglect bedding – make it seasonally appropriate and breathable, and replace old pillows that have lost their support.

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