A study of more than one million adults found that six or seven hours of sleep a night beat eight. Those on shorter nights had a lower death rate.

The findings appeared in Archives of General Psychiatry in 2002, from research run between 1982 and 1988. They challenge advice millions have followed for decades.

The work came from the University of California, San Diego School of Medicine and the American Cancer Society. It was the first large study to compare people while accounting for factors such as age, diet, exercise and smoking.

The best survival rates were found among those sleeping seven hours. A group on eight was 12 per cent more likely to die within six years than the seven-hour group.

Even people getting five hours lived longer than those on eight or more. Dr Daniel Kripke, the study's first author, said six and a half hours a night was safe, with no health reason to sleep longer.

The study did not explain why longer sleep was linked to higher mortality. It also noted insomnia is not the same as short sleep, with occasional sufferers facing no raised death rate.

What Ancestral Sleep Patterns Show

A new review in the journal Brain Medicine challenges the idea that all adults need eight hours. Its authors say sleep in pre-industrial societies shows 'substantial flexibility'.

That suggests today's norm of one long, unbroken night is not evolutionarily fixed. Researchers have studied hunter-gatherer groups with little exposure to artificial light, The Conversation reports.

A study in Current Biology found people in Tanzania, Namibia and Bolivia slept between 5.7 and 7.1 hours. Long daytime naps did not make up the gap.

Without alarm clocks, they woke close to dawn whatever the day. An analysis from Royal Society Publishing puts pre-industrial sleep at 6.4 hours.

Modern patterns are delayed against the natural light and dark cycle, it adds. Artificial light from lamps and phones is blamed, and Nature research says it delays the body clock.

Why Individual Sleep Needs Differ

Health status and biological sex also shape how much rest a person needs. Many large studies place the sweet spot below eight hours, The Conversation notes.

A Nature study used brain and body scans alongside molecular measures to track organ ageing. It found that the healthiest range for biological ageing was between 6.4 and 7.8 hours, depending on sex and organ.

Work-hour guidance for nurses from the US Centers for Disease Control and Prevention says most adults need seven to eight hours of good-quality sleep. The amount needed may be an inherited trait.

Genetic studies have found mutations in a small number who sleep six hours or less and stay well. Most who claim to need little sleep are simply pushing themselves.

They then struggle to stay awake during the day, the guidance warns. Over time, they risk obesity and chronic illness.

Why Eight Hours Is Not Fixed

The National Sleep Foundation recommends seven to nine hours for adults aged 18 to 64. Those over 65 may need slightly less, seven to eight.

Some do well at the lower end of a range, while others need every minute of the upper limit. An extra hour either side may suit some adults.

Sleeping too little has been linked to weight gain, weaker immunity, high blood pressure and depression. Mood, attention span and job performance suffer too.

Guidance suggests recording bedtimes and waking times in the second week of a relaxing fortnight off. Those needing several cups of coffee daily may not be sleeping well.

Feeling refreshed on waking is the sign to look for. As The Conversation puts it, the eight-hour rule 'is clearly too simple'.

Frequently Asked Questions

  • What is the ideal amount of sleep for adults?
    The ideal amount of sleep for adults is generally between 6.4 and 7.8 hours, depending on individual needs.
  • Why might longer sleep be linked to higher mortality?
    The study did not explain why longer sleep is linked to higher mortality, but it suggests that more sleep is not necessarily better for health.
  • How do ancestral sleep patterns differ from modern ones?
    Ancestral sleep patterns show substantial flexibility, with people in pre-industrial societies sleeping between 5.7 and 7.1 hours, often waking close to dawn without alarm clocks.
  • What factors influence individual sleep needs?
    Health status, biological sex, and genetic traits can influence how much sleep a person needs.
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