For many people, the thought of eight hours’ sleep is a glorious and unattainable dream. No aches and pains keeping you awake, no racing thoughts, worries and anxious glances at the alarm clock as the night ticks down to a weary dawn. 

The pandemic only made our sleep patterns worse, with a study last year from the University of Southampton showing that the number of people experiencing insomnia rose from one in six to one in four. Even now, it’s a rare person who still doesn’t feel like they could do with a couple more hours every night.

But according to a new American study, hitting this “magic number” of eight hours might not be so good for our health – quite the opposite, in fact. The study seemed to suggest that sleeping for this long might even hasten the onset of Alzheimer’s disease. 

Scientists from the Sleep Medicine Centre at Washington University recruited 100 elderly people and monitored their sleep while giving them tests for alertness and cognitive ability. They found that sleeping more than 7.5 hours could increase a person’s likelihood of worsening brain function – a classic symptom of Alzheimer’s.

“Our study shows that there is a middle-range or ‘sweet spot’ for total sleep time where cognitive performance was stable over time,” says Dr Brendan Lucy, lead author of the study.

Sophie Bostock is a sleep scientist with a PhD in health psychology from UCL.“This study is consistent with other studies in the general public that suggest too much or too little sleep is linked to worse cognitive function,” she says. Guy Leschziner, the author of Nocturnal Brain and a professor of neurology and sleep medicine at Guy’s Hospital, agrees.

“The association between sleep deprivation and cognitive decline has been keenly studied, and there is a scientific rationale for limited sleep being detrimental to brain health,” he says. But, adds Leschziner, the link between long sleep duration and Alzheimer’s is less intuitive. 

“It may be less of a function of sleep duration and more related to the nature of sleep itself,” he says. “Anything disrupting sleep quality may result in a longer sleep requirement, but it is also possible that early dementia may cause fundamental changes in the way we sleep.” 

In other words, the subjects of the study may already have been suffering from cognitive difficulties before they started the trial.

So what, then, is the sleep “sweet spot” – and is it really possible to sleep for “too long”, as well as not long enough?

There is received wisdom about how many hours a person “should” sleep. Until this latest study, the oft-quoted “magic number” was eight hours. Two years ago, there were rumours of new government guidelines which were going to suggest we should sleep between seven and nine hours, but these never materialised.

This figure hasn’t been pulled out of the air: there have been several studies backing it up. A 2019 paper published in Harvard Health concluded that a minimum of seven hours is recommended for good health. The research was based on hundreds of studies that followed peoples’ long-term experience of heart disease, diabetes, and mental health difficulties. Those who slept between seven and nine hours were typically at lower risk of future ill-health.

The time you need to spend unconscious varies with age. One journal detailed that 18- to 60-year-olds “need” seven to eight hours, 61- to 64-year-olds “need” seven to nine, and the over-65s drop an hour again (for some unspecified reason).

This followed a study in the peer-reviewed scientific journal Sleep, where 1.4 million adults were followed by researchers at Warwick University and the University of Naples medical school. The research found that six hours’ sleep or fewer were associated with a 12 per cent increased risk of premature death.

In contrast, however, other studies have shown a link between oversleeping and bad health.

The Warwick and Naples study also found a 30 per cent rise in risk of death for people who slept nine hours or more, possibly because they may have underlying medical or social problems. Interestingly, the report concluded that while short sleep may represent a cause of ill health, long sleep is believed to represent more of an indicator of ill health.

A closer look at the Washington study revealed that those in the middle range of 5.5 and 7.5 hours showed no signs of a worsening of cognitive function. “The study also suggests that sleep quality may be key, as opposed to total sleep,” says Dr David Holzman, co-author of the study.

Sleep quality is the measurement of how well you’re sleeping – in other words, whether your sleep is restful and restorative. Sleep quality is trickier to measure than sleep quantity, and is based on factors such as sleep latency (the time it takes you to fall asleep), and how often you wake up during the night. 

High-quality rest also depends on a smooth, repeated progression through the nightly sleep cycle.

“It doesn’t matter if you wake up during the night,” says Sophie Bostock. “Your sleep goes in 90-minute cycles, and people tend to wake up more as they get older.”