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. Even people of the same age can need very different amounts of sleep
Current sleep duration recommendations, from the National Sleep Foundation in the US, reflect these individual differences. It says that adults aged 26 to 64 should get between seven and nine hours of sleep per day, but that sleeping between six and ten hours may be appropriate for some adults. That is a long way from the one-size-fits-all eight-hour rule
When you sleep matters
But there is more to sleep than the hours we get. An important feature of modern sleep patterns is not that they are shorter than they once were, but that they are delayed relative to the natural light-dark cycle. Most of us do not wake naturally before dawn. Many of us have short sleep during the working week and longer catch-up sleep on work-free days
Artificial light is recognised as the main driver of this shift. It changes our sleep habits and delays our body clock, which controls the timing of sleep and wakefulness
About the authors
Derk-Jan Dijk is a Professor of Sleep and Physiology and Director of Surrey Sleep Research Centre, University of Surrey. Anne Skeldon is a Professor of Mathematics, Head of School, School of Mathematics & Physics, University of Surrey. This article was first published by The Conversation and is republished under a Creative Commons licence. Read the original article
Irregular sleep is also common, particularly among shift workers and university students. Irregular sleep is linked to poorer health. But if you lose sleep during the week, catching up on some of it at the weekend appears to be better than not catching up at all – even though it makes your sleep pattern less regular. So regularity matters, but so does the total amount of sleep you get over a week or longer.
The eight-hour rule is clearly too simple. People need different amounts of sleep, and when we sleep matters too. But one simple message remains: get enough sleep when you can, wherever you can, and don’t worry too much about the occasional bad night
