Working the night shift and jet lag might be doing more harm to your body than making you feel tired.

According to a new study published in the Proceedings of the National Academy of Sciences, shift work can cause changes at the molecular level. Researchers were surprised to discover how severe the body is damaged when the sleep-wake cycle is altered.

"Over 97% of rhythmic genes become out of sync with mistimed sleep, and this really explains why we feel so bad during jet lag, or if we have to work irregular shifts," co-author Dr Simon Archer of the University of Surrey said in a statement.

In the study, researchers gradually shifted the sleeping patterns of 22 participants from a normal pattern to that of a night-shift worker. Researchers placed participants in a controlled environment without natural light, delaying their sleep-wake cycle by four hours each day. The team then collected blood samples to measure the participants' rhythms of gene expression.

Researchers found there was a sixfold reduction in the number of genes displayed during a 24-hour period. Gene regulators linked to transcription and translation – which suggests “widespread reduction” to biological processes in the body.

"It's chrono-chaos. It's like living in a house. There's a clock in every room in the house, and in all of those rooms those clocks are now disrupted, which of course leads to chaos in the household," researcher Professor Derk-Jan Dijk from the University of Surrey told the BBC, describing how the rhythms of the heart, kidneys and brain could effectively be running at different times when a person’s sleep cycle is disrupted.

This isn’t the first time shift work has been linked to negative health risks like heart attacks and diabetes.

"We of course know that shift work and jet lag is associated with negative side effects and health consequences,” Dijk said. "They show up after several years of shift work. We believe these changes in rhythmic patterns of gene expression are likely to be related to some of those long-term health consequences."

While it may be unrealistic to stop night shifts altogether, researchers say the study sheds light on how sleep functions and how sleep-wake schedules influence biological processes including aging.

“I don’t think in our society we can’t do without shift work, but we can start to think about how we mitigate the impact and understand how it affects our bodies,” Dijk told The Telegraph.