The findings could lead to new treatments for jet lag, insomnia and seasonal depression.

BBC Science Correspondent Christine McGourty: "Sleep disorder may be hereditary"

The researchers from the University of Utah say people who cannot stay awake much after 8.30pm and who wake up around 5.30am may be suffering from a disorder called "familial advanced sleep-phase syndrome".

This means the person's body clock is three to four hours faster than normal.

Writing in Nature Medicine, the scientists, led by Dr Louis Ptacek, say they found 29 people in three families who suffered from the disorder.

In one family, a grandmother, daughter and grandchild all had the disorder which is not the same as simply finding it difficult to stay in bed in the morning.

Dr Ptacek said most people with the disorder do not see their doctor about it, but some find it causes them problems.

Jet lag

Scientists hope that by studying the syndrome they may be able to identify the gene responsible.

This could lead them to the protein it produces to cause the shortened time clock which, in turn, could be used to formulate new drugs to treat jet lag and other conditions.

However, they admit that it will take some time to isolate the gene.

Dr Ptacek compared the task to "looking for a spelling error in a single book "in a library full of 80,000 books.

Experts in circadian rhythms - the rhythms that regulate sleep and wakefulness - say the research is exciting.

David Earnest of Texas A&M University said: "This is the first time that anyone has identified a genetically inherited trait that involves the expression and control of the circadian rhythm in humans. That's really critical."