The eight-hour sleep rule is outdated. Here’s how much you really need

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For decades, the answer to “how much sleep does an adult need?” has been simple: eight hours. The idea dates back to the mid-19th century when the notion of eight hours of work, eight for recreation and eight for sleep first took hold.

But growing evidence suggests that eight hours may not be the ideal amount of sleep for everyone, as a new narrative review in Brain Medicine reminds us.

To find out how much people naturally sleep, researchers have studied hunter-gatherer communities whose lifestyles are closer to those of our ancestors and who have little or no access to artificial light. In one early study, researchers found that people in Tanzania, Namibia and Bolivia slept for between 5.7 and 7.1 hours a night. These short sleep durations can’t be attributed to traffic noise, social media use or too much artificial light at night.

The short nights also seemed to be enough: people did not make up for them with long daytime naps. Without an alarm clock, they tended to wake close to dawn, regardless of the day of the week.

Exactly how long people are found to sleep in pre-industrialised societies varies depending on how sleep is measured, where people live, the season and other environmental factors. But one recent analysis estimated the average to be 6.4 hours.

So, is eight hours too much? These studies make an important assumption that the amount people naturally sleep is the amount they need. That assumes that hunter-gatherers are free from stress, busy schedules and other pressures on sleep. But is that really the case?

We also don’t know whether people in these societies experience the same health problems linked to too little sleep that are seen in modern societies. So their shorter sleep may not be as strong an argument against eight hours as it first appears.

But other evidence has emerged from large studies looking at the relationship between sleep and health in industrialised societies.

If we define optimal sleep as the amount linked to the best health, many large studies suggest the sweet spot may be less than eight hours. One recent study used brain and body scans and molecular measures to look at how sleep relates to the ageing of different organs. It estimated the optimal amount to be between 6.4 and 7.8 hours, depending on the person’s sex and the organ being studied.

Many studies find that both too little and too much sleep are linked to poorer health, including heart disease, stroke, cognitive decline and obesity. But scientists are still debating whether too much sleep causes poor health or is instead an early sign of it. Either way, these findings challenge the idea that everyone needs exactly eight hours and suggest that consistently sleeping for much longer may be worth paying attention to.

However, all these estimates of optimal sleep duration are based on averages, and the average person does not exist. When given enough opportunity to sleep, studies find that young adults sleep longer than healthy older adults.