Researchers identified the gene variant that affects virtually the entire population responsible for up to an hour a day of your tendency to be an early riser or night owl.
The discovery also finds this genetic variant helps determine the time of day a person is most likely to die.
The surprising findings, could help with scheduling shift work and planning medical treatments, as well as in monitoring the conditions of vulnerable patients.
"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. It also influences the timing of acute medical events like stroke and heart attack," said first author Andrew Lim.
"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," adds Lim, Assistant Professor in the Division of Neurology at the University of Toronto.
Lim and his colleagues compared the wake-sleep behaviour of individuals with their genotypes. These findings were later verified in a group of young volunteers.
They discovered a single nucleotide near a gene called "Period 1" that varied between two groups that differed in their wake-sleep behaviour. At this particular site in the genome, 60 per cent of individuals have the nucleotide base adenine (A) and 40 per cent have guanine (G).
As there are two sets of chromosomes, in any given individual, there's about a 36 per cent chance of having two As, a 16 per cent chance of having two Gs, and a 48 per cent chance of having a mixture of A and G at this site.
"This particular genotype affects the sleep-wake pattern of virtually everyone walking around, and it is a fairly profound effect so that the people who have the A-A genotype wake up about an hour earlier than the people who have the G-G genotype, and the A-Gs wake up almost exactly in the middle," said Clifford Saper, Professor at Harvard Medical School.
Expression of the Period 1 gene was lower in the brains and white blood cells of people with the G-G genotype than those with the A-A genotype, but only in the daytime, which is when the gene is normally expressed.
"Virtually all physiological processes have a circadian rhythm, meaning that they occur predominantly at certain parts of the day. There's even a circadian rhythm of death, so that 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 Saper in a statement.
