We become biologically older when our bodies are under stress, but younger again when we recover, according to a study that analysed people’s DNA when they had emergency hip surgery, severe covid-19 or were pregnant.

“This recovery suggests we have the machinery to be able to rewind the clock back at least a little bit,” says at Duke University in North Carolina, who co-led the study with at Harvard University.

We normally measure age by the number of birthdays – so-called chronological age. But people can be biologically older or younger than their chronological age depending on factors such as whether they smoke or .

To measure biological age, researchers have developed “epigenetic clocks” that analyse patterns of markers on DNA called methyl groups that correlate with age.

White, Gladyshev and their colleagues used these clocks to assess the impact of three types of stressful event on biological age. In each case, they analysed DNA from blood samples that were collected at multiple points in time from participants in previous studies.

In the first analysis, the team found that the biological age of nine people with an average age of 81 rapidly increased when they had emergency surgery to repair a broken hip, but returned to pre-surgery levels over the following week.

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Next, the team measured the biological age of 29 people with an average age of 60 while they were hospitalised with severe covid-19 and after they were discharged. The biological age of female participants fell after they were discharged, but that of male participants didn’t, possibly because from the disease, says White.

Finally, the team compiled data from four studies that included more than 200 women who were pregnant, which is known to put stress on the body. Their biological age increased over the course of the pregnancy, but, by six weeks after delivery, it had returned to below the levels seen in early pregnancy. No transgender men or non-binary people were included in this part of the study.

The researchers also used epigenetic clocks to measure the biological age of mice before, during and after pregnancy and found the same pattern.

In another experiment, the team showed that young mice experienced a sudden increase in biological age after they were surgically joined to older mice so that blood from the older mice flowed into their own circulations. This was reversed after they were surgically separated from the older mice.

The idea that biological ageing speeds up during stressful events but reverses afterwards is consistent with a previous study that found that people’s grey hairs sometimes regain their original colour after they recover from psychologically stressful events like marital separation.

However, at the University of Sydney in Australia says that even though there may be short-term fluctuations in biological age, the overall trend still continues towards becoming older. “Your grey hair may regain some colour, but it isn’t going to revert to the hair you had as a 10-year-old,” he says.

Nevertheless, now we know that biological ageing can at least reverse slightly, it raises the possibility of being able to develop therapeutics to drive this reversal further, says White.