The findings, published in the November issue of Annals of Neurology, focus mainly on how gene mutations can account for differences in circadian rhythms — including entire families who wake up extremely early or can't stay awake after 8 p.m.
While the data from the long-term study may help with work scheduling and planning medical treatments, it is the time-of-death forecast that has captured most attention.
"There's even a circadian rhythm of death," says Clifford Saper, chief of neurology at Harvard Medical School's Beth Israel Deaconess Medical Centre in Boston and co-author of the study, "so that in the general population, people tend on average to be most likely to die in the morning hours.
"Sometime around 11 a.m. is the average time."
The researchers, led by first author Andrew Lim, assistant professor in neurology at the University of Toronto, were initially looking at how the circadian clock regulates the body in terms of preferred sleep times, times of peak cognitive performance and the timing of many physiological processes.
But when investigators went back and looked at participants who had been in the longest — some for more than 15 years starting at age 65 — and who had since died, they found a startling fact.
The same genotype that governed people's sleep-wake patterns predicted the time of death.
People with the AA or AG genotype died just before 11 a.m., like most of the population, but those with the GG genotype on average died at just before 6 p.m., said a release from the Beth Israel Deaconess Medical Centre.
"So there is really a gene that predicts the time of day that you'll die," says Saper. "Not the date, fortunately, but the time of day."