Understanding what governs people's wake-sleep cycles has important medical implications for dealing with insomnia, jet lag, shift work and depression, the scientists report in the September issue of the journal Nature Medicine.
Researchers studied people who have a shorter than normal wake-sleep cycle. Regardless of work schedules or social pressures, these folks just can't stay up much later than 8:30 p.m. and they tend to wake up around 5:30 a.m.
It's called "familial advanced sleep-phase syndrome" because it shifts the normal wake and sleep pattern forward by three to four hours.
Louis Ptacek and colleagues found 29 people in three families with the disorder. One family included a grandmother, daughter and grandchild all with the same sleep disturbance.
The normal population contains plenty of "morning lark" and "night owl" people who function better at different times of day, but people with this syndrome are not simply morning people, Ptacek emphasized.
Most creatures seem to operate on a biological clock approximately synchronized to a 24-hour day. This rhythm controls a variety of daily biochemical and behavioral cycles including fluctuations in sleep and wakefulness.
For people with this syndrome the cycle is shorter and the constant conflict between their body clock and coping with the rest of the world leads to their shifted sleep-wake rhythm.
"These aren't diseases per se, and most people just live with this sleep pattern and never see a doctor about it," Ptacek said. Some cope with it fairly well, but it can be disabling for others.
By studying the family relationships the scientists found that the disorder is inherited in a way common to other inherited traits caused by a single gene, such as eye color.
So now Ptacek's team is working to identify the specific gene responsible. Finding the gene could lead them to the protein it produces to cause the body's time shift a discovery that could then lead to the development of drugs to treat not only the sleep disorder, but jet lag and other conditions.
"Presumably, this protein is just one of the gears in the (body) clock. By studying one gear we can get a handle on the other gears around it and we may get an insight about why they have this trait and how the normal clock functions," Ptacek said.
His report was greeted with enthusiasm.
"I think this is a really exciting finding," said David Earnest of Texas A&M University, who studies the cell biology of the circadian clock that regulates sleep and wake patterns.
"This is the first time that anyone has identified a genetically inherited trait that involves the expressin and control of the circadian rhythm in humans. That's really critical," Earnest said.