How well people perform on tests after being deprived of sleep depends in part on their genes, new research suggests. After staying awake all night, individuals with a long version of the PER3 gene only scored half as well on cognitive tests as subjects with a short version.
What is more, the greatest differences in performance were seen during the small hours - the time when most tiredness-related accidents happen and when shift-workers have most trouble staying awake.
"It may be there are people who are genetically predisposed against shift work," says Malcolm von Schantz, at the University of Surrey, UK. But he emphasises that gene tests should not be used to discriminate against such individuals. It is very possible that carriers of the long PER3 gene have advantages at other times, he notes.
Von Schantz and his colleagues recruited 12 volunteers who carried two copies of the long version of PER3 and another dozen with two copies of the short version. Using electroencephalogram machines to record brain activity, researchers monitored the participants' sleep patterns.
Word games
The recordings revealed that although both groups slept for about the same amount of time, subjects with the short PER3 gene spent roughly 15% of this time in deep sleep, also known as slow-wave sleep. Their counterparts with the long PER3, meanwhile, spent about 22% in this restorative sleep stage.
When subjects woke up from their first night of sleep in the lab, they had to remain in bed. Researchers removed clocks from the room and kept light and temperature levels steady to avoid giving subjects a sense of time. Over the course of the next 40 hours participants had to stay awake and periodically complete cognitive tests such as word memory games on a laptop.
In the first 18 hours of these tests, both groups performed the same on the test. But by morning, after being awake for 24 consecutive hours, those with the long PER3 managed to score only 50% as well as those with the short version.
So even though people with the long form of PER3 normally get more deep sleep, they actually perform worse on cognitive tests when deprived of sleep. The findings suggest some people need more deep sleep than others.
Larking about
"The findings are very interesting and novel," says Chiara Cirelli at the University of Wisconsin in Madison, US. She adds that the study "nicely links" the need for deep sleep with performance during sleep deprivation.
In the UK, approximately 10% of people have two copies of the long version of PER3. Roughly 45% of the country's population has two copies of the short PER3 gene, whilst another 45% have both versions of the gene. An earlier study of 500 individuals found that people who carry the long version are "larks" and generally go to bed early, while those with the short form are "owls" and typically stay up late.
Previous experiments have suggested that the protein made from PER3 can activate other genes, but the exact role it plays in the cell remains unknown.
For this reason, researchers remain stumped as to how PER3 variations might influence sleep or cognitive performance. "We don't actually have a biological explanation for this," says von Schantz.
Nevertheless, the team note that the variations may be "an important marker for individual differences in sleep and susceptibility to sleep loss, which are major causes of health problems and accidents in our society".
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