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 tobe the ideal amount - but some experts now say it's too much, and may actually beunhealthy.

We all know that getting too little sleep is bad. You feel tired, you may be irritable, and it can contribute toobesity, high blood pressure, diabetes, and heart disease, doctors say. But too much sleep? You don't oftenhear people complaining about it.

However, research carried out over the past 10 years appears to show that adults who usually sleep for lessthan six hours or more than eight, are at risk of dying earlier than those sleep for between six and eighthours.

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 ofWarwick, has analysed 16 studies, in which overall more than a million people were asked about theirsleeping habits and then followed up over time.

Cappuccio put the people involved into three broad groups:

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, comparedto 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 ofalcohol 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 usingsleeping 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 healthproblem that is not yet showing in other symptoms.

So, it's not the long sleep that is causing the increased mortality risk, it's the hidden illness.

The sleep lab at Warwick University

But not everyone agrees. Prof Shawn Youngstedt of Arizona State University carried out a small studyinvolving 14 young adults, persuading them to spend two hours more in bed per night for three weeks.

They reported back that they suffered from "increases in depressed mood" as Youngstedt puts it, and also"increases in inflammation" - specifically, higher levels in the blood of a protein called IL-6, which isconnected with inflammation.

The participants in the study also complained about soreness and back pain. This makes Youngstedtwonder whether the problem with long sleep is the prolonged inactivity that goes with it.

He has now been carrying out an experiment where long-sleeping and average-sleeping adults are askedto spend an hour less in bed each night. The results will be published soon, he says.

Anyone studying sleep has to contend with a number of difficulties. One is that it's often not possible tomeasure sleep very accurately.

"We tend to rely on very simple methods of asking people on average how many hours they sleep a night.It has to be taken with a pinch of salt," says Cappuccio.

"Naturally, you have to rely on your memory, and you don't know if you're reporting time in bed or timeasleep and whether you're accounting for naps, and so forth."

Apparently we have a general tendency to overestimate how long we've been asleep. And when it comes toquality of sleep, all experts seem to agree it could affect your health, but it's even harder to measure thanhow long you sleep.

Another caveat is that babies, children and teenagers all have different sleep requirements than adults.

But if it's the case that less than six hours of sleep is too little for an adult, and more than eight hours is toomuch, what is the ideal amount - what do our bodies want?

As we've reported before, there is a lot of evidence to suggest that until the late 17th Century people didnot sleep in one long uninterrupted stretch, but in two segments, separated by a period of one ortwo hours in which they prayed, read, chatted, had sex, smoked, went to the toilet or even visitedneighbours.

That may be more natural than the current tendency to sleep - or try to - in one stretch.

Putting this question to one side, and focusing on the total number of hours spent asleep, Cappuccio saysthree-quarters of people in the Western world sleep between six and eight hours a night on average, therange associated with the best results in terms of length of life.

But can we say that eight hours are better than six?

The magic number, according to Dr Gregg Jacobs, of the Sleep Disorders Center at the University ofMassachusetts 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 ofadults in the US

"The typical adult today [in that poll] reports seven hours of sleep. And that actually seems to be themedian sleep duration in the adult population around the world. That suggests there's something aroundseven 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 nohard evidence that extra time asleep, or just lying down and relaxing, is going to kill you.

Listen to More or Less on BBC Radio 4 and the World Service, or download the free podcast.

Subscribe to the BBC News Magazine's email newsletter to get articles sent to your inbox.