Watching TV before you go to bed gives you a bad night's sleep and can lead to chronic health problems, scientists have claimed.

Psychiatrists were surprised to find watching television appeared to be the most dominant pre-sleep activity and sleep patterns were based around schedules rather than sunset or biological factors.

Researchers think watching TV is replacing vigorous activity like sports and the lack of exercise results in poor sleep quality.

The US study of 21,475 adults found TV viewing was far and away the most dominant pre-sleep activity - accounting for almost 50 percent of pre-bed time.

One in three Brits have sleep problems making it one of the most common complaints doctors hear. As a result sleep deprivation is becoming a national problem, warn health experts.

Sleep is so important because it allows the brain to recover from the rigours of the day. Not getting enough has been found to increase the risk of obesity, heart disease and depression.

Dr Mathias Basner, of the Division of Sleep and Chronobiology, University of Pennsylvania, said: "While the timing of work may not be flexible, giving up some TV viewing in the evening should be possible to promote adequate sleep.

"According to our results, watching less television in the evening and postponing work start time in the morning appear to be the candidate behavioural changes for achieving additional sleep and reducing chronic sleep debt."

Co-researcher Dr David Dinges, Professor of Psychology in Psychiatry at the university, said: "Given the relationship of short sleep duration to health risks, there is concern that many Americans are chronically under-sleeping due to lifestyle choices."

The government is keen to tackle these health issues, efforts doomed to failure unless getting enough sleep is made a priority as well.

The latest study, presented at the Associated Professional Sleep Societies' annual meeting in Illinois, examined the activities over 15s undertook two hours before and after bed time.

The researchers said sleeping less than seven to eight hours daily impairs alertness but despite this up to four in ten Americans do not achieve the recommended amount.