Our body's circadian clocks may be responsible for more than making us early birds or night owls.
New research has found that the gene variant affecting our body clock can also predict the time of our death.
The study was published in the November 2012 issue of the Annals of Neurology and was conducted by the Department of Neurology at Beth Israel Deaconess Medical Center.
"The internal 'biological clock' regulates many aspects of human biology and behaviour, such as preferred sleep times, times of peak cognitive performance, and the timing of many physiological processes," author Andrew Lim told Harvard News. "It also influences the timing of acute medical events like stroke and heart attack."
The study's authors examined the brains of 1,200 participants after they had died and found that there is a "circadian rhythm of death".
"In the general population people tend on average to be most likely to die in the morning hours. Sometime around 11 am is the average time," said Clifford Saper, another of the study's authors. "So there is really a gene that predicts the time of day that you'll die. Not the date, fortunately, but the time of day."
This finding is based on the most common genotypes, A-A or A-G, which around 84 per cent of individuals have. The G-G genotype, on the other hand, is more likely to die just before 6pm.
The discovery of a single genotype that dictates the time of day we are likely to die was chanced upon while Lim and his colleagues were working on another study, Harvard News reports. The original study, which examined 1,200 healthy 65-year-olds annually until death, set out to understand whether there were links between sleeping troubles, ageing and Alzheimer's disease.
Lim then learned that the same participants in his study had their DNA genotyped for another analysis. The colleagues from the two studies collaborated and compared the sleep-wake patterns of the participants with their genotypes [differences in their genetic make-up].
They discovered that the differences were linked to a genotype variation that "affects the sleep-wake pattern of virtually everyone walking around," and leads to some people waking up, on average, a full hour before others. They also found it affects our body's natural rhythms, meaning we have different levels of alertness at different times of the day.
The study's authors hope that this knowledge can help us to better understand human behavioural rhythm.
"Previous work in twins and families had suggested that the lateness or earliness of one's clock may be inherited and animal experiments had suggested that the lateness or earliness of the biological clock may be influenced by specific genes" said Lim.
They also say that the results may "facilitate individualised scheduling of shift work, medical treatments, or monitoring of vulnerable patient populations."