Researchers have identified the cause of a sleep disorder which causes people to nod off early and wake fully alert before dawn.
FASPS patients tire early in the evening
A mutant gene,CKIdelta, is to blame for the condition, familial advanced sleep phase syndrome (FASPS).
The findings may lead to new treatments for FASPS, and disturbed sleep patterns caused by jet lag or night shift work.
The University of California, San Francisco and Howard Hughes Medical Institute study features in Nature.
Most people with FASPS are able to live normal lives, and some are proud of being able to rise early, and get a lot done while everything is quiet.
Familial advanced sleep phase syndrome
The internal clock of most people operates in just over a 24-hour time period
The clock of those with FASPS advances an average of 45 minutes a day
If left unchecked, it would continue to speed up
Due to the social demands and needs of those with the disorder, it is kept in check to a relative degree
However, some people are distressed at living out of sync with everyone's else's daily schedule.
People with FASPS do not seem to sleep any more or less than other people - they just sleep at different times.
Researcher Dr Louis Ptacek said: "Often, they have adjusted and accommodated their jobs to match their ability.
"But others are bothered by being out of phase with the rest of the world."
The researchers studied three generations of a family in which five members were affected by FASPS, waking up on average at around 0400 every morning.
They found 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.
When the mutant version of the gene was inserted into mice they became early risers - mimicking the effect in humans.
However, when the same version of the gene was inserted into fruit flies it did the opposite, lengthening their daily rhythm, and turning them into late risers.
The results suggest the gene produces different effects depending on its setting.
The researchers also found that all six people they studied had not only FASPS, but also asthma and migraine headaches.
Dr Ptacek said this raised the possibility that the symptoms were all part of the same syndrome.
The researchers hope their work will lead to a greater understanding of the complex controls regulating the daily, or circadian rhythms of humans, and other animals.
Dr Ptacek told the BBC News website: "The discovery of the gene opens the window just a crack, but it could let in a lot of light for probing the neurobiology of the brain.
"It is possible that insights into how the human clock works will allow treatment of other circadian problems like jet lag or shift work."
Dr Ying-hui Fu, who also worked on the study, said: "Evidence suggests that circadian rhythms may have a fundamental role in numerous behaviours.
"As the enzyme produced by the gene modulates many proteins, we may test for its impact on learning and memory too."
Dr Melissa Hack, who chairs the British Sleep Society, said: "There is evidence that there is some inherited genetic element to sleep patterns - for instance some people can cope much better with sleep deprivation than others.
"However, genetics is probably just one of many factors controlling sleep. Some people can learn how to change their sleep patterns, and we have also had a lot of success in treating disturbed sleep using light therapy."
Dr Andrew Cummin, a sleep expert at Charing Cross Hospital, said the findings potentially had wide implications.
"Understanding how these mutations speed the internal clock opens the door to the possibility of regulating our own body clocks using drugs.
"If we can find a way to do this, not only will we be able to help patients with phase shifts but there may, one day, even be a cure for jet lag."