Sleep experts explain which metrics genuinely reflect your health, which are little more than educated guesses, and why your sleep score may be causing more anxiety than insight
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Just because you can, doesn’t mean you should. This might be a phrase you’d more likely associate with acts of indulgence than tracking health data, but it turns out that despite the masses of information (and money-making potential) there is within sleep data, not all of these points are reliable or useful.
If you find sleep scores are keeping you up at night, along with health anxiety, which is a very real thing, it might provide some comfort to know what sleep experts actually consider to be useful data. This way, you can try not to read too much into the rest of your, uh, rest.
While you can track these approximately without any wearables, Private GP Dr Hugh Coyne says that “the most useful measurements from a sleep wearable are how long you sleep, when you sleep, and how consistent those patterns are from one night to the next.”
Sleep duration, as you imagine, is about aiming for seven to nine hours of sleep a night (though each person is different, this is the sweet spot), sleep consistency is about aiming to sleep and wake roughly the same time each day.
A 2023 UK Biobank study of more than 60,000 adults found that the most regular sleepers had up to 48 per cent lower all-cause mortality risk than the least regular sleepers, “and this was a stronger association than sleep duration alone produced in the same data.” Says longevity expert and Medical Director Dr Memee Ahmad
Falling asleep and waking at roughly the same time each day, including weekends, is also said to carry many benefits when it comes to sleep hygiene, cortisol production, and circadian rhythm regulation.
As wearables can track this accurately, this is the most important set of data to factor into life. “Sleep stage percentages and proprietary sleep or readiness scores warrant far less attention.” Says Dr Coyne.
With data, the main issue both Dr Coyne and Dr Ahmad stress is to do with how reliable the data likely is. Wearables tend to rely on “indirect signals” to track sleep stages, Dr Coyne says for example, and so it is important that any data we use is reliable first and foremost.
Heart-related data is, after sleep duration and timing, where these devices can be reliably accurate. When it comes to resting heart rate (RHR) Dr Ahmad says, “Your lowest heart rate overnight is a solid marker of cardiovascular fitness. A rising trend over several days, above your personal baseline, often flags illness, alcohol, stress or overtraining before you consciously feel it.”
Heart rate variability (HRV), meanwhile, measures the variation in time between heartbeats: “this reflects how balanced your autonomic nervous system is.” Dr Ahmad says. Increasing your HRV tends to mean you are improving your capacity to deal with stress. “Comparing your HRV to someone else’s is not useful; the number is highly individual and should be read as a trend against your own data.” Dr Ahmad adds.
Some devices measure skin temperature, not core body temperature, Dr Coyne warns, but paying attention to temperature can be useful – both as a metric, and as a way to improve sleep hygiene, and is measurable at home easily. “Our core body temperature naturally drops to initiate sleep and rises again toward waking.” Dr Ahmad says.
If you find your temperature is elevated, whether due to alcohol, an incoming illness, or simply the weather, doing what is needed to cool the body down can help to mitigate bad sleep and help you maintain quality.
Regardless of the data set, we all have nights where our sleep is a little wonky. This can be due to anything from outside noises to the food we ate. Look for the rolling average “across at least one to two weeks, including weekends.” Coyne says. If you find, for example, you have consistently slept worse this month than last month, that can suggest there’s something to look into. Weekends are important because we need to factor in any changes that are due to lifestyle – whether it is a lie-in or a late night.
Data is useful, but nothing is generally as useful as how you feel. “If the device says you slept poorly, but you feel rested and function well during the day, the number should not overrule your experience.” Dr Coyne says.
Essentially, sleep data is tracking sleep, not causes of fatigue or energy. If the data is fine but you don’t feel great, that can suggest inaccuracy within the data, or an issue that isn’t being tracked, such as iron deficiencies. Blood tests and conversations with your doctor can be useful in these instances.
Of course, the same applies to feeling good despite bad data. And beyond the data, there are symptoms that suggest all is well. Dr Ahmad says that, for example, “waking close to your natural time without needing multiple alarms, feeling alert within around 20 to 30 minutes of getting up, having stable energy through the day, not having hard afternoon crashes, and feeling that your mood, stress, and appetite are steady, are all very good signs.”
It is fair to take the reverse of these, such as morning grogginess, needing a dozen alarms, relying on caffeine, having a dry mouth, snoring, or testosterone-related issues as potential warnings for a poor night’s sleep (though these are far easier to realise than the things that are going well).
Tracking data then, is not always an exact science. Dr Coyne says, “I would largely ignore the precise amount of deep sleep or REM sleep reported by the device. Wearables infer sleep stages from movement, heart rate and other indirect signals. They do not measure brain activity in the way that a formal sleep study does.”
Often, the most important sleep-related metrics for overall health aren’t reliably tracked, and so they shouldn’t be the numbers we focus on improving – working on those we can reliably track, however, tends to provide the benefits we were looking for all along
