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.

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

But not everyone agrees. Prof Shawn Youngstedt of Arizona State University carried out a small study involving 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 is connected with inflammation.

The participants in the study also complained about soreness and back pain.

This makes Youngstedt wonder 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 asked to 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 to measure 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 time asleep 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 to quality of sleep, all experts seem to agree it could affect your health, but it's even harder to measure than how 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 too much, 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 did not sleep in one long uninterrupted stretch, but in two segments, separated by a period of one or two hours in which they prayed, read, chatted, had sex, smoked, went to the toilet or even visited neighbours.

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 says three-quarters of people in the Western world sleep between six and eight hours a night on average, the range 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 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.