Forget counting hours of sleep - this underrated fitness tracker feature helped me develop better rest habits

If you've ever woken up feeling sluggish, tired...and confused because you've got an excellent sleep score after clearly a bad night's sleep, you're not the only one. Not every tracker records heart rate variability (HRV), yet it's one of the most interesting metrics on a fitness tracker.

HRV is the tiny difference in time between each heartbeat. The nervous system, which is made up of the sympathetic branch (fight or flight) and parasympathetic (rest and digest), controls it.

"Although your heart may be beating 60 times per minute, those beats are not perfectly evenly spaced. That variability provides useful information about how the autonomic nervous system is responding to stress and recovery," says Dr Mark Kovacs, a performance physiologist, researcher, and sport technology expert.

I started tracking my HRV when testing some of the best sleep trackers, and I've done it every night for the past year. It's given me insights into my sleep habits and health that I'd not seen before.

What is heart rate variability (HRV)?

Heart rate variability (HRV) is the time between heartbeats, says Dr Kovacs. "In general, a higher HRV relative to your baseline suggests great 'rest and recovery' activity, while a meaningful reduction can indicate your body is stressed or hasn't fully recovered," he explains.

HRV is recorded during REM sleep, and many sleep trackers will pick up on it, recording it alongside other important metrics, like how long you slept for, how long it took you to fall asleep, and any breathing irregularities.

"Sleep duration tells us how long somebody slept, and a sleep score generally combines several estimates about their sleep. HRV gives us something slightly different: insight into how the nervous system and cardiovascular system responded during that period of recovery," says Dr Kovacs, who leads the Kovacs Institute.

I started looking at my heart rate variability

I've been using the Oura Ring for well over a year - first the Oura Ring 4 and now the Oura Ring 5. These rings include HRV in the Readiness Score, but I drilled into the HRV feature alone. This is where I found the most interesting information. Every morning, while drinking my tea and with the previous day's activities fresh in my mind, I take a look at my overnight data.

For me, drinking alcohol before 7 pm doesn't impact my recovery that much, but after 9 pm, I tend to suffer. The same goes for exercise and eating. If I get back from a run late in the evening, after 8 pm, then I feel it overnight. Eating past 10 pm disrupts my sleep, and I don't wake up feeling as rested.

Any expert will tell you that alcohol, exercise, and eating late at night can impact your sleep, but this timing information is personal to me, gathered over a year of testing and experimenting. The results completely changed how I spend my evenings when rest is a priority.

Along with my evening plans, I use this data to plan my exercise routine for the week, more so than my general sleep score. If I've done a hard workout, my HRV will naturally be lower as my body struggles to recover. If that's the case, I'll plan an easier workout or slower run in the days afterwards, and I'll feel better for it. If I don't feel so tired, I'm more likely to exercise, and this has helped me be consistent in my exercise routine.

I'm also keeping a close eye on my HRV as we get closer to winter. A sudden drop in your overnight HRV can be a sign that you're getting ill before you even have symptoms. I saw it a few times at the beginning of the year, and while it's too late to stop anything, it gave me a day or so to stock up on zinc or rearrange my social plans for the week.

With this information, I felt I could make better choices for my weekly routine, with actually very little thinking required.

Having this data is "genuinely useful", says Dr Kovacs. "It encourages us to connect the number with what's happening in our lives. Think: What has changed in the past 24 to 72 hours? Did I train much harder? Did I sleep poorly? Am I under unusual stress? Do I feel as though I'm becoming ill?"

My chosen sleep tracker is the Oura Ring 5, but I've also tested my heart rate variability on the Ultrahuman Ring Pro, the Garmin Venu 4, and the Fitbit Air. All are different prices and have various functions, but they can all record HRV.

What does low heart rate variability mean?

Low HRV means something is up with your body. It could be that glass of wine too close to bedtime, late-night eating, or something more serious. "It can be indicative of poor sleep quality, since sleep fragmentation and brief awakenings can cause sudden spikes in heart rate, leading to a higher HRV," says Dr Chelsie Rohrscheib, a neuroscientist, sleep expert, and the head of sleep at Wesper.

"Individuals with certain sleep disorders may also see atypical HRV," she says. "Insomniacs may find their HRV is lower than expected, whereas individuals with sleep apnoea may have repeated spikes and dips in HR throughout the night."

One low HRV reading may not be a problem, says Dr Kovacs. "What becomes more interesting is when you see a consistent change over multiple nights, and it corresponds with other information, such as poorer overall sleep, unusually high resting heart rate, and reduced fatigue."

Smart devices, like fitness trackers, are not sensitive enough to detect or diagnose sleep disorders. But changes in heart rate, HRV, oxygen levels, or sleep patterns detected on one of these devices can provide useful clues, says Dr Rohrscheib. You can then take this information to your doctor.

If you are concerned about your sleep health, make an appointment with your GP or a dedicated sleep specialist.

How to improve heart rate variability

1. Focus on consistency

If you want to improve your overnight recovery, consistency is key, says Dr Kovacs. Try to get to bed and wake up at the same time every day.

"A late night followed by sleeping in for several hours isn't necessarily equivalent to maintaining a consistently good sleep routine. Think about sleep as something you train, just like physical fitness," he says.

2. Do a variety of exercise

Regular exercise is one of the best ways to improve your cardiovascular health, says the doctor, but challenging workouts add stress to the body and are harder to recover from than more moderate workouts.

"A good weekly program should combine cardiovascular exercise, resistance training and easier recovery days," he says. "If overnight HRV has been noticeably lower than normal for several days and you also feel unusually fatigued, that may be a useful signal to reduce training intensity temporarily rather than pushing through another very hard session."

3. Wind down before bed

“What happens during the final few hours before sleep can have a surprisingly large influence on overnight recovery," says Dr Kovacs, so create a bedtime routine that limits all kinds of stress.

"Alcohol is one of the first things I would look at because even when alcohol appears to help someone fall asleep, it can negatively affect overnight physiology and recovery," says the doctor. "Large meals immediately before bed, psychological stress and very late intense exercise can also keep the body in a more activated state."