Why a long night's sleep may be bad for you

Many of us try, but often fail, to get eight hours' sleep each night. This is widely assumed to be the ideal amount - but some experts now say it's too much, and may actually be unhealthy.

We all know that getting too little sleep is bad. You feel tired, you may be irritable, and it can contribute to obesity, high blood pressure, diabetes, and heart disease, doctors say. But too much sleep? You don't often hear people complaining about it.
However, research carried out over the past 10 years appears to show that adults who usually sleep for less than six hours or more than eight, are at risk of dying earlier than those sleep for between six and eight hours.
To put it more scientifically, there is a gradual increase in mortality risk for those who fall outside the six-to-eight-hour band.
Prof Franco Cappuccio, professor of cardiovascular medicine and epidemiology at the University of Warwick, has analysed 16 studies, in which overall more than a million people were asked about their sleeping habits and then followed up over time.
• those who said they slept less than six hours a night
• those who said they slept for between six and eight hours
• those who said they slept for more than eight hours

His analysis showed that 12% more of the short sleepers had died when they were followed up, compared to the medium sleepers.

However, 30% more of the long sleepers had died, compared to the medium sleepers.
That's a significant increase in mortality risk, roughly equivalent to the risk of drinking several units of alcohol per day, though less than the mortality risk that comes from smoking.
But can it really be true that getting nine hours' sleep is worse for you than getting five?
There are different ways of looking at this.
Cappuccio was aware of the possibility that people sleeping too long might be depressed, or might be using sleeping pills. He corrected for this, though, and found the association was still there.
His own theory is that people who sleep for more than eight hours sometimes have an underlying health problem that is not yet showing in other symptoms.
The magic number, according to Dr Gregg Jacobs, of the Sleep Disorders Center at the University of Massachusetts Medical School may actually be seven.
"Seven hours sleep keeps turning up over and over again," he says.
He points, for example, to the National Sleep Foundation's annual poll of a random sample of adults in the US
"The typical adult today [in that poll] reports seven hours of sleep. And that actually seems to be the median sleep duration in the adult population around the world. That suggests there's something around seven hours of sleep that's kind of natural for the brain."

But if you enjoy sleeping, spend a lot of time in bed and feel good, you're probably just fine. There's no hard evidence that extra time asleep, or just lying down and relaxing, is going to kill you.