Nurses are among the workers who have to work at night. Are women affected differently than men by night shifts?

Do men and women have different circadian rhythms?

New research may provide answers as to why some people tolerate being awake at night better than others.

Staying awake at night and being able to sleep during the day goes against our natural circadian rhythm, but some people cope better than others.

Recently, researchers have found evidence that biological differences between the sexes can affect the circadian rhythm of both humans and mice.

There are many indications that women and men have internal clocks that are set a little differently, according to a background article in the journal Science about some of the more recent research in the field.

"It’s exciting and quite new that we can say something more about the mechanisms behind the differences in circadian rhythms," says Andrea Rørvik Marti, a PhD candidate in psychology at the University of Bergen (UiB) after reading the article. She did not contribute to it herself.

But she’s quick to add that while gender can affect circadian rhythms, a lot of other things may do so too. There are many more similarities than differences between the sexes.

Your biological clock is found throughout the body

Nobel laureates Michael Rosbach, Jeffrey C. Hall and Michael W. Young found that genes control our inner clock, which ticks away depending on light and darkness.

This clock is found throughout the body, Marti says, but the brain is in the driver’s seat.

"The brain is the conductor of an orchestra. Every single cell in our body contains genes that control that clock. They are the musicians," she says.

"When you have to be awake and active at an unexpected time, you confuse these clocks. The big question is why some people have problems as a result, and why others deal with it quite well," she says.

While some people think night shifts are fine, other people never get used to them.

«There are divergent findings, and we need more experimental studies on both animals and humans before we can conclude that gender actually affects the biological clock,» says circadian rhythm researcher Andrea Rørvik Marti.

The role of sex hormones

Women are more likely to be morning people and seem to tolerate nocturnal disturbances better than men, according to some research. But why?

There are few studies on the differences between women's and men's 'orchestras', but animal studies suggest that sex hormones may affect the 'conductor', namely the brain.

A study shows that the female sex hormone oestrogen is sensitive to light. When the researchers removed oestrogen in mice, they were less reactive to light.

This means that mice with oestrogen were more likely to be woken by the light, even when they should have been asleep.

"This can be a good thing if you have to travel through several time zones or if you have to perform at night," Marti says. "But not when you have to go to bed and sleep the morning after your night shift."

The two American researchers who wrote the Science review wondered if women cope with shift work better than men because they are programmed by nature to take care of children, who can wake quite a lot at night.

But this is currently only a hypothesis.

Another study, however, shows that women perform worse on tests than men when their circadian rhythms have been disturbed.

"There are divergent findings, and we need more experimental studies on both animals and humans before we can conclude that gender actually plays a role in our biological clock," says Marti.

Night shifts can be harmful to health

Some studies have found that night shifts are associated with an increased risk of cancer, cardiovascular disease and diabetes. Other studies have found no connection with breast cancer.

However, it’s not at all clear that health problems associated with night shifts are due to biological differences between the sexes.

"There can be many different reasons, from physical and mental health to life situations," Marti says

For example, it’s mostly men who do shift work offshore, while nurses who do shift work are most often women.

"These are very different situations. Offshore, you have no children to pick up at kindergarten and you can focus on resting when you have free time," Marti said.

Large individual differences

Marti herself studies circadian rhythms. In a recent study published in the journal Scientific Reports, she reported how well male rats performed after being kept awake in a rat running wheel three nights in a row.

"There were large individual differences between rats of the same sex and age," she says.

There were variations in both whether the rats managed to stay awake during a night shift and whether they passed the test the next day, when they had to find a platform under water to stand on.

Body temperature is a measure of where in the circadian cycle the body is. The rats that performed better had a body temperature that indicated that their internal clock was stable even though they were not allowed to sleep.

"Research has long focused on the idea that sleep is important for performance. But being able to keep your internal clock stable is also important," Marti said.

Translated by Nancy Bazilchuk


Seán T. Anderson and Garret A. FitzGerald: Sexual dimorphism in chronobiology has implications for the health of our 24-hour society. Science, 2020. Doi: 10.1126 / science.abd4964. Summary.


Read the Norwegian version of this article at

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