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.
Interestingly the researchers noticed that all six people they studied had asthma and migraine headaches as well as FASPS, which raises the possibility that the symptoms are all part of the same syndrome says Dr Ptacek. He also says that the discovery of the gene could provide insights into how the human clock works and lead to a greater understanding of the complex controls regulating the daily, or circadian rhythms of humans, and other animals.
Dr Ying-hui Fu, one of the researchers says the evidence suggests that circadian rhythms may have a fundamental role in numerous behaviours as the enzyme produced by the gene modulates many proteins and could be tested for its impact on learning and memory.
Dr Melissa Hack of the British Sleep Society, says there is evidence of some inherited genetic element to sleep patterns - for instance some people can cope much better with sleep deprivation than others but genetics is probably just one of many factors controlling sleep. Some people can learn how to change their sleep patterns. They have had a lot of success in treating disturbed sleep using light therapy.