More than two-thirds of the population are not getting the recommended eight hours of sleep a night, with many people lying awake worrying about money, a survey has claimed.

Money was the main factor keeping people up at night, with more than half of insomniacs citing worries over finances as their biggest nocturnal concern.

Two-thirds of people aged between 25 and 34 said they were stressed about general money problems. A third of those questioned in the YouGov survey managed only six hours of sleep, or less, a night.

The rising cost of electricity and gas was the biggest concern for more than a third of all those unable to sleep. Half of all those over-55s who could not sleep said they were worried they were unable to afford to keep their homes warm. Five of the big six energy providers have increased tariffs since January. Fuel poverty now affects 4.5 million people in Britain, with bills costing more than 10 per cent of their net income.

Fear of crime was not a big concern, with only 15 per cent of people saying it was a main factor in keeping them awake at night.

That figure fell to 7 per cent in the 18 to 24, and 35 to 44, age brackets. The over-45s were getting the least sleep, the survey found, with only 24 per cent saying they slept for eight hours. Fuelling the perception that students lie in bed all morning, those aged between 18 and 24 reported getting the most sleep, with 46 per cent enjoying eight hours sleep or more.

The full findings of the survey will be published today. It found that 68 per cent of people get fewer than eight hours of sleep each night.

But it is wrong to believe all adults need eight hours of sleep a night, according to Professor Jim Horne, the director of Loughborough University's Sleep Research Centre. He said: "It's nonsense. Lots of people report having more or less sleep than the average. It may all be down to genes, and what people are accustomed to."

The US National Sleep Foundation suggests seven to nine hours a night is advisable for adults. Its advice states: "In the past century, we have reduced our average time in sleep. Though our society has changed, our brains and bodies have not. Sleep deprivation is affecting us all and we are paying the price."

But Professor Horne said: "The test of insufficient sleep is whether you are sleepy in the day or if you remain alert through most of the day. In a nutshell, if you sleep for eight hours a night, go to work and find yourself lolling and drooling on the keyboard, you aren't getting enough."

While the amount of sleep needed by an adult varies between individual s, several studies showed sleep deprivation can lead to dramatically reduced brain function in the short term and severe medical conditions, including heart problems, in the long term. However, studies often yield divergent results. A previous YouGov survey claimed the nation was shunning sex in favour of a good night's sleep.

A study by the University of California involving one million participants, showed a clear association between long duration sleep and high mortality rates.

The average Briton sleeps more than eight hours a night, according to the Office for National Statistics.