Scientifically Speaking | The sleep monitor on your wrist might just save your life

Consumer devices like fitness trackers are great at tracking how long you sleep and identifying patterns in different sleep stages, like REM and light sleep

I've worn a fitness tracker on my wrist that also monitors my sleep every night for nearly a decade. Through thousands of nights of data, I’ve come to realise that there’s a pattern. The nights I sleep poorly correlate directly with drops in my mood and measurable fitness the next day. My personal observations, though anecdotal, now mirror what sleep scientists are discovering through massive studies of sleep-tracking data.

These everyday devices from smartphones to fitness trackers to smart rings have turned our bodies into streams of medical data.

We are living through a revolution in medicine. Your doctor can now monitor your heart rhythm through your watch, adjust your insulin pump remotely, or spot early signs of illness through patterns in your daily movements. Rural patients can connect with specialists hundreds of kilometres away through video calls, while computer systems can analyse medical scans in seconds. This flood of personal health data, collected continuously as we go about our lives, gives doctors new tools to catch problems early and tailor treatments to each patient's needs.

These everyday devices from smartphones to fitness trackers to smart rings have turned our bodies into streams of medical data. The same devices that count steps and track workouts also reveal hidden patterns in our sleep. These patterns provide early warning signs of heart problems, mood disorders, and rapid ageing long before other symptoms appear. Three major studies published recently show just how much these electronic sleep diaries can tell us.

At Vanderbilt University, Evan Brittain and colleagues analysed sleep records from 6,785 people wearing Fitbit devices, tracking their rest patterns over 4.5 years. Their study in Nature Medicine captured a total of 6.5 million nights of sleep, which is more than all previous laboratory sleep studies combined. The researchers found that keeping regular sleep hours matters as much as getting enough sleep. They also found that people with erratic bedtimes faced higher risks of depression, anxiety, and heart problems, even when they slept the same number of total hours.

The Vanderbilt study uncovered two important patterns in how sleep affects our health. First, keeping regular sleep hours matters a lot. When people's sleep times bounced around by an extra hour each night, their health risks climbed: hypertension rose by 56%, anxiety disorders by 55%, and the risk of major depression jumped by 75%.

But crucially, the amount of sleep matters too. The researchers mapped out what happens when people get too little or too much sleep, using 6.8 hours as their baseline. Those who routinely slept just 5 hours faced serious health risks. Their chances of developing high blood pressure rose by 29%, while their risk of depression increased by 64%.

Surprisingly, sleeping longer wasn't better. People who regularly slept 10 hours faced even greater risks. Their chances of developing high blood pressure rose by 61%, and their risk of depression more than doubled, jumping by 163%. This pattern, where both too little and too much sleep raises health risks, creates what scientists call a "J-shaped" curve, showing that sleep is best in moderation.

Modern sleep trackers do much more than count hours of rest. They map out different stages of sleep through the night, and these patterns tell a rich story about our health. Take heart rhythms, for example. The Vanderbilt study found that the type of sleep matters as much as the amount. People who got more deep sleep and REM (rapid eye movement) sleep showed healthier heart patterns.

Even small changes made a difference: each percentage point increase in REM sleep was linked to a 14% reduction in the odds of developing atrial fibrillation, a dangerous heart rhythm disorder. Light sleep showed the opposite effect, with more time in light sleep stages associated with higher heart risks.

Your body's daily rhythms matter too. Jae Kyoung Kim's team at the Korea Advanced Institute of Science and Technology studied this in detail, tracking 168 patients with mood disorders. Published in NPJ Digital Medicine, their research revealed how shifts in sleep timing warned of mood changes weeks in advance. By analysing 36 different features of sleep and daily patterns, they could predict incoming depression with 80% accuracy and manic episodes with 98% accuracy from nothing more than data from phones and wearables.

Your body runs on daily rhythms that control everything from hormones released to body temperature. The timing of these rhythms, called the circadian phase, proved especially important for younger adults. Those under 45 whose daily patterns ran just 25 minutes later than usual showed signs their bodies were ageing faster than their peers. This timing shift mattered less in older adults, suggesting that regular schedules might particularly benefit younger people's health.

The connection between daily rhythms and ageing emerged even more clearly in research from ETH Zürich, also published in NPJ Digital Medicine. Filipe Barata's team studied 80,000 adults and developed a new way to calculate how fast someone's body was ageing based on their daily patterns.

When someone's calculated biological age ran ahead of their actual age, health risks rose sharply. Each year of accelerated ageing meant 8-12% higher mortality risks and up to 14% higher chances of age-related diseases. The brain seemed especially sensitive to these rhythm disruptions. Irregular daily patterns predicted nearly triple the risk of developing conditions like dementia or Alzheimer's disease.

So, what should we make of all these recent studies in terms of advice to follow? It’s a good idea to set regular sleep and wake times (even on weekends). It might also be a good idea to aim for about seven hours of sleep, since both shorter and longer sleep times carry health risks. And lastly, we should pay attention to gradual changes in sleep patterns, since they might warn of health changes before other symptoms appear.

Consumer devices like fitness trackers are great at tracking how long you sleep and identifying patterns in different sleep stages, like REM and light sleep. But they can't measure brain activity as accurately as the equipment used in medical sleep labs. While no consumer device perfectly captures every aspect of sleep, they reliably track patterns over time. This matters because sleep changes often precede health problems. The message from these recent studies is clear and convincing. Understanding your sleep patterns and maintaining healthy sleep habits helps to protect your health.

Anirban Mahapatra is a scientist and author, most recently of the popular science book, When The Drugs Don’t Work: The Hidden Pandemic That Could End Medicine. The views expressed are personal.