Some bedheads who chronically sleep through their alarms may now have a new excuse: They're genetically predisposed to get up late. A new study in the 15 June issue of Sleep, reveals the first correlation between a gene involved in our internal body clock and a tendency to go to bed late.
Many natural rhythms, such as preferred mealtimes and body temperature, are at the mercy of the circadian clock, a system involving at least nine genes that keep our bodies in sync with the 24-hour, light-dark cycle. Evidence has been building that slight person-to-person variations in these genes can also help explain variability in sleep cycles. Molecular biologist Simon Archer of the University of Surrey, U.K., and his colleagues noticed previous studies documenting two versions of one of these genes, called Per3. One produces a protein 20% shorter than the other. Both proteins react with chains of others in the brain to control hormones, such as melatonin, that regulate our sleep-wake cycle.
To test whether the two variants affect the body's daily cycle, they set up a stand at an exhibition held in London's Science Museum in fall 2001. The researchers collected cheek cell DNA samples from 484 volunteers, who also filled in standard questionnaires to determine how much of a night owl or early bird they are. Archer's colleagues at hospitals in London and the Netherlands also collected blood from 16 sufferers of Delayed Sleep Phase Syndrome (DSPS), a condition characterized by a pathologically extreme evening preference. DSPS sufferers are often unable to fall asleep before 4:00 in the morning.
The group found that the shorter protein-producing gene had a 76% frequency among the 35 most extreme evening-type museum volunteers, compared with only 58% among the 35 most extreme morning-type volunteers tested, says Archer. Further tests showed that 75% of the DSPS sufferers had inherited two copies of the shorter Per3 gene, one from each parent, whereas none had inherited two copies of the longer variant.
The study adds considerable weight to the idea that the time of day we naturally wake up and go to sleep is in our genes, says Fred Turek, a neurobiologist at Northwestern University in Evanston, Illinois. Picking a career with hours unsuited to your natural cycle, he adds, could be like trying to fit "a round ball into a hole for a square peg."