The disrupted sleep cycles associated with shift work and jet lag have profound effects on the human body, according to a new study, which found that genes expressed themselves differently when subjects’ sleep cycles were altered.

Researchers at the Sleep Research Centre at the University of Surrey in the U.K. found that study subjects who were put on a 28-hour sleep cycle expressed far fewer genes than when they were on their normal 24-hour cycle.

For the study, 22 volunteers had their sleep patterns extended by four hours each day so at the end of three days they were sleeping during the day. Regular blood tests showed “a massive impact” on gene expression over the days the subjects’ sleep cycles were extended to 28 hours, study co-author Simon Archer said in a statement.

As the study got underway, the researchers found that nearly 6.5 per cent of the subjects’ genes followed a 24-hour circadian expression, a finding consistent with other research. But after the three 28-hour days, only one per cent of the subjects’ genes remained on the 24-hour rhythm.

“That’s quite a reduction,” Derk-Jan Dijk, a sleep physiologist at Surrey, said in a statement.

According to Dijk, a number of the body’s normal rhythms “have a reduced amplitude in general when you’re not sleeping at the right time of day.”

The researchers found that genes that normally express themselves during the day and genes that normally express themselves at night were affected during the study.

Despite the changes, the study found that what the researchers called the body’s “master clock,” the suprachiasmatic nucleus (SCN) of the hypothalamus, continued to keep a 24-hour rhythm. The researchers came to this conclusion after finding that melatonin levels remained consistent in the subjects over the course of the study.

“So what we’re starting to see is that some aspects of rhythmicity in some parts of the brain are still intact, but in other parts of the body the rhythmicity is disrupted,” Dijk said. It is those disturbed bodily rhythms that lead to the physiological effects associated with shift work or jet lag, he said.

What remains unclear, according to Dijk, is how exactly the changes in sleep timing lead to changes in gene expression. And a key limitation of the study is whether it would have produced the same results if the researchers had changed the subjects’ sleep cycles over a different time frame.

However, what is clear is that altered sleep cycles do have a major impact on gene expression, said neurobehavioural geneticist Valter Tucci of the Italian Institute of Technology in Genova.

“It’s not by chance that evolution has favoured the development of sleep at a particular time,” Tucci, who was not involved with the U.K. study, said in a statement. “Be careful going against what evolution has given us, we might screw up other systems.”

The findings were published Tuesday in the journal Proceedings of the National Academy of Sciences.