Fragmented sleep can contribute to the development of cancer and make the disease more malignant and invasive, according to a study published by the journal Cancer Research ahead of World Cancer Day 2014.

Fitful sleep can also weaken the immune system’s ability to fight off cancer, especially in the early stages. The study on lab rats marks the first time a direct link has been found between irregular sleep and cancerous growths, while revealing the mechanism responsible.

Researchers from the University of Chicago and the University of Louisville conducted the study, led by Dr. Fahed Hakim, a pediatric specialist in lungs and sleep from Rambam Medical Center in Haifa who spent two years at the University of Chicago for the project.

According to Hakim, the study is striking considering the relatively low importance many people attribute to sleep – something that should take up roughly one-third of our lives.

“When sleep is fragmented and we wake up many times a night – whether it’s someone who has to do so for work or an asthma patient who wakes up to cough – we’re unaware of the significance,” Hakim told Haaretz.

“Studies of fragmented sleep have linked it to immediate effects on daily life like fatigue and irritability, but we’ve seen in this study that the effects are much greater. Over time, fragmented sleep weakens the immune system, and in this case, contributes to the development of cancer.”

Hakim and his colleagues studied two different sets of rats; one group was allowed to sleep normally while the others had their sleep disrupted. After a week, the rats were injected with two different types of cancer cells. In 9 to 12 days, both groups began to develop cancerous growths.

When researchers measured the growths after four weeks, they found significant differences. Growths in the sleep-deprived rats were twice as large on average as growths in the rats that were allowed to sleep normally.

This trend was observed in both types of cancerous growths in the experiment. In a follow-up experiment, when cancer cells were injected into the thigh muscles of sleep-deprived rats, the growths were not only larger, they were much more malignant and invasive.

“Injecting cancer cells into the thigh muscle convinced us that the growths were more malignant and invasive than the growths in rats that slept normally,” Hakim said. “Under normal circumstances, solid growths of this kind ... are stationary and remain encapsulated. We saw that in sleep-deprived rats, the growths weren’t stationary and penetrated lung-muscle tissue and bones.”

The differences between the two groups are linked to immune cells known as tumor-associated macrophages, TAMs, or as Hakim puts it, “immune cells that fight off foreign invasions. These cells provide a way to measure the quality of the immune system, and they can respond in a variety of ways according to the chemical signals they’re given.”