Poor sleep may be fuelling the global rise in under-50s being diagnosed with cancer, two large studies suggest.

The number of younger people diagnosed with the disease has risen by almost 80% in three decades. Worldwide cases of early-onset cancer increased from 1.82m in 1990 to 3.26m in 2019, while cancer deaths among people in their 40s, 30s or younger rose by 27%.

Experts are still trying to understand the reasons behind the increase. However, research presented at the world’s largest cancer conference, the American Society of Clinical Oncology’s annual meeting in Chicago, suggests irregular sleeping patterns in younger people may be a contributing factor.

Two studies led by MD Anderson Cancer Center in Houston, Texas, one of the world’s leading cancer research organisations, analysed health data for more than 18 million adults in the US aged between 18 and 50.

Researchers found that people with poor sleeping patterns were more likely to develop early-onset bowel, breast, uterine or ovarian cancer. In some cases, under-50s diagnosed with insomnia were three times more likely to develop cancer within five years.

“These findings suggest that sleep disruption may represent a clinically relevant, potentially modifiable risk factor in early-onset cancer risk stratification and warrants further investigation,” the researchers said.

Identifying the causes of rising cancer rates among younger adults has become a global health priority. More than 1 million under-50s die from cancer each year, according to research published in BMJ Oncology.

Experts not involved in the studies welcomed the findings but said further research was needed to better understand the link between insomnia and early-onset cancer.

Claire Coughlan, the clinical lead at Bowel Cancer UK, said bowel cancer was still more common in over-50s, but there was a growing body of evidence globally that cases were increasing in younger people.

“We don’t know exactly why this is yet, but researchers currently believe it may be due to genetics and lifestyle factors,” she said. “This study’s conclusion that insomnia may be a potential risk factor in early-onset colorectal cancer could go some way to helping us understand the reasons behind this increase. The findings of this study are worth exploring further.”

Dr David Garley, a GP and director of the Better Sleep Clinic in Bristol, England, said the impact of insomnia on someone’s risk of other health conditions, including cancer, was of “growing interest”.

However, he cautioned that the studies had identified an association rather than proof that sleep disruption caused cancer in under-50s. There could be several reasons for the link, Garley said.

“There are possible causes from the physiology that arises from poor sleep, but also the fact that when sleep-deprived it’s difficult to live a healthy life – there is more drinking, more obesity, less exercise, more smoking etc – and these may be what causes any possible increased risk.

“If you’re sleeping badly, we know that one of the roles of sleep is restoring the immune system. We’re understanding more about the role of infective agents in the development of cancer.

“So if your immune system takes a bit of a hit, then I suppose it would make sense that your risk of some cancers may increase. But it also might be the other way around. It could be that if you have cancer, and it’s not yet clinically obvious, it could be causing some change in how you sleep.”

Megan Winter, a health information manager at Cancer Research UK, said: “These studies explore whether there could be a link between insomnia and some types of cancer in people under 50, but more research is needed, particularly looking at people over longer periods of time, before we can draw any conclusions.”

In the meantime, she added, people could reduce their risk of cancer by not smoking, maintaining a healthy weight and staying safe in the sun.