A sleep disorder which makes people nod off early and wake fully alert before dawn is now thought to be caused by a mutant gene.
Researchers at the University of California, San Francisco and Howard Hughes Medical Institute, have identified a gene,CKIdelta which they say is to blame for the condition, familial advanced sleep phase syndrome (FASPS).
The results could lead to new treatments for FASPS, and disturbed sleep patterns caused by jet lag or night shift work.
People with FASPS do not seem to sleep any more or less than other people - they just sleep at different times and some are distressed at living out of sync with others' daily schedule and are bothered by being out of phase with the rest of the world.
Researcher Dr Louis Ptacek says many of these people have often adjusted and accommodated their jobs to deal with their disorder.
A family with three generations, of which five members were affected by FASPS, were studied by the researchers, all waking up on average at around 0400 every morning.
It was found that the FASPS patients carried a mutated version of the CKIdelta gene, which controls production of a protein thought to have a key role in regulating the body clock.
The mutant version of the gene when inserted into mice made them become early risers - mimicking the effect in humans, but when it was inserted into fruit flies it did the opposite, lengthening their daily rhythm, and turning them into late risers. This suggests, depending on its setting, the gene produces different effects.