Are you getting enough shut-eye? The usual advice is that we need to get eight hours of sleep every night, for the sake of our health.

But now, research suggests the impact of getting less sleep than that has been exaggerated. In fact, getting as little as six hours sleep a night is probably fine for physical and mental health. So do we need to rethink the current obsession with getting enough sleep?

The goal of eight hours a night is recommended in many sources of health advice, with the NHS website saying that adults need from seven to nine hours of sleep a night.

Warnings include that lack of sleep leads to heart attacks, mental health problems – even Alzheimer’s disease.

But the science on just how much slumber we need is more complex than the health messaging would have us believe.

It is impossible to investigate this question with the most rigorous kind of medical research, a randomised trial, because few people would be willing to deliberately change their sleep patterns for long periods for the cause of science.

Instead, researchers may study people’s average sleep duration and figure out how that correlates with their health. This approach has long relied on volunteers filling in surveys about their sleep patterns – but that tends to be inaccurate. “There are all kinds of problems with self-reports,” said Evan Brittain, a professor of medicine at Vanderbilt University Medical Center in Nashville, Tennessee.

The science has recently been put on a firmer footing by a new breed of study, where people wear activity trackers like Fitbits, which can detect when people are asleep.

In the latest study, Professor Brittain’s team asked those taking part in a larger health research project to share their Fitbit data, with almost 7,000 people agreeing. And over the next five years, their medical records were used to track if anyone developed conditions such as high blood pressure, which can lead to heart attacks and strokes.

The average amount of sleep that people got each night was 6.7 hours. “Most people are not getting anywhere near eight hours of sleep,” said Professor Brittain. “And most of those people were perfectly healthy.”

How long people slept was linked with certain health conditions. For high blood pressure, as well as depression and anxiety, people were at higher risk if they had either very low or very high amounts of sleep; in other words, there was a U-shaped curve charting the relationship between sleep duration and ill health.

The lowest risk of these three conditions was in people getting slightly under seven hours of sleep a night, and the risk slowly rose as sleep duration became either more or less than this. But there was a broad sleep duration range that was linked with optimum health, with any duration from six to eight hours a night giving similar health outcomes.

“Our findings support the notion that seven hours of sleep may be the middle of the healthy range for adults, rather than the floor,” the research team said in their paper, published in the journal Nature Medicine.

However, when it came to severe obesity and sleep apnoea – when the airways narrow causing someone to wake up multiple times a night – the longer someone slept, the less likely they were to have these conditions.

That may be because both severe obesity and sleep apnoea can cause people to have less sleep, said Professor Brittain.

The findings are not the first to challenge the idea that we should strive for eight hours of shut-eye to stay in good health. Last year a large study found that both short and long sleep durations were linked with lower brain volume but again, the optimum amount of sleep was a relatively low 6.5 hours a night.

“The seven-hour sweet spot is now replicated across a number of diseases, functions, health variables and even mortality,” said Professor Anders Fjell at the University of Oslo in Norway, who carried out that analysis.

It is likely that we naturally vary in the amount of sleep we need, and that our bodies usually make sure we get enough sleep, unless prevented by night-time disruptions like a wakeful baby, said Professor Fjell. “There are a lot of individual differences in sleep need.”

Older studies that used questionnaires rather than activity trackers may have overestimated sleep durations, said Professor Michael Chee, director of the Centre for Sleep and Cognition at the National University of Singapore. “Most people will give their time in bed rather than the time they’re actually asleep.

“This is a landmark study because of its approach to marrying wearable data with electronic health records.”