Researchers have found genetic factors play a major role in sleep disorders such as severe snoring and involuntary leg jerking.

The findings are based on a study of almost 2,000 pairs of female twins by the Twin Research Unit at St Thomas' Hospital, London.

Details will be published in the journal Twin Research.

Previously it had been thought that problems such as snoring were probably due to factors such as being overweight, or sleeping in the wrong position.

Among the conditions examined by the St Thomas' team were obstructive sleep apnoea (OSA) and restless leg syndrome (RLS).

OSA is a severe form of snoring in which the throat narrows to the point where breathing becomes momentarily impossible.

The condition, which affects approximately 24% of men and 9% of women aged 30 to 60, can destroy the ability to sleep soundly, and leaves sufferers exhausted.

It has been blamed for road traffic accidents where people have fallen asleep behind the wheel.

RLS causes an irritating, non-painful sensation in the legs, which compels suferers to keep moving them, and keeps them awake throughout the night.

Researcher Dr Adrian Williams said: "Sleep disorders are surprisingly common and it is increasingly recognised that they can have a devastating impact on sufferers' everyday lives - they are no laughing matter."

The research, on 1,937 pairs of identical and non-identical twins, found:

Approximately 50% of the variation in sleeping disorder symptoms can be pinned on genetic factors.

For disruptive snoring it was 42%, daytime sleepiness 45%, restless legs 54% and legs jerking 60%.

The figures took into consideration other factors, such as smoking and obesity, which can also increase the risk of snoring and daytime sleepiness.

Professor Tim Spector, director of the twin unit, said: "These results suggest a substantial genetic contribution to the symptoms of both obstructive sleep apnoea and restless legs syndrome and that could be good news for people who suffer from these conditions if the genes responsible can be identified.

"One reason the genes for disruptive sleep may have persisted is that poor sleep patterns make people gain weight and retain fat.

"These genes may have helped our ancestors through periods of famine and the Ice Age."

Professor Spector said there was also a possible evolutionary advantage in people carrying genes that made them sleepy and alert at different times to their peers.

"If the whole human race had fallen asleep at three in the afternoon, then it might have had dire consequences for the human race," he said.

Marianne Davey, director of the British Snoring & Sleep Apnoea Association, said: "This study has given some excellent insights into the possible genetic influences of snoring and sleep apnoea and from this we may be more inclined to examine familial background when taking a sleep history."

However, she said a follow-up study was required to establish whether the same conclusions applied to men.

The twin unit was set up in 1992 to look at the role that genes play in the development of rheumatic diseases in older women and has now expanded to lok at most common diseases, behaviours and traits.

If you are a twin (identical or non-identical) aged over 15 who would like to volunteer to join the 10,000 twins on the register of the Twin Research Unit at St Thomas' Hospital, call 020 7188 5555.