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A good night's sleep is extremely important for both physical and mental health. Previous findings have suggested that a lack of good sleep contributes to fatigue, moodiness and impaired cognitive functions the morning after. In a new study, researchers found another potential side effect of poor-quality sleep. According to the researchers, fragmented sleep, which is when people wake up numerous times throughout the night, can contribute to cancer growth.

"It's not the tumor, it's the immune system," said study director David Gozal, MD, chairman of pediatrics at the University of Chicago Comer Children's Hospita;. "Fragmented sleep changes how the immune system deals with cancer in ways that make the disease more aggressive."

For this study, Gozal worked with researchers from the University of Chicago and the University of Louisville and conducted a series of experiments on mice. In one set of mice, the researchers disrupted their sleep by using a quiet, motorized brush every two minutes. In the other group of mice, the researchers did not disturb their sleep. Both sets of mice lived in these conditions for an entire week.

After day seven, the researchers injected both groups of mice with one of two tumors, which were TC-1 or 3LLC. After nine to 12 days, the injected mice all developed tumors. The team examined the mice after four weeks. The researchers discovered that the tumors present in the mice that had fragmented sleep were doubled the size of the tumors in the other group of mice. In the follow-up study, the researchers injected the tumor in the thighs of the mice, where tumors are generally contained. The researchers found that the mice with fragmented sleep had tumors that were more aggressive.

"In that [thigh] setting, tumors are usually encased by a capsule of surrounding tissue, like a scar," Gozal said according to a press release. "They form little spheres, with nice demarcation between cancerous and normal tissue. But in the fragmented-sleep mice, the tumors were much more invasive. They pushed through the capsule. They went into the muscle, into the bone. It was a mess."

The researchers explained that sleep appeared to affect the mice's cells from the immune system. The cells, known as tumor-associated macrophages (TAMs), can build up at the tumor site. TAMs can respond in numerous ways. TAMs labeled MI can trigger an immune response that aims to destroy tumor cells. TAMs labeled M2 end up suppressing the immune system, which can promote tumor growth. In these sets of mice, the researchers found that the mice with good-quality sleep had more M1-type TAMS in their tumors. The other set of mice, however, had primarily M2-type TAMs.

"This study offers biological plausibility to the epidemiological associations between perturbed sleep and cancer outcomes," Gozal said. "The take home message is to take care of your sleep quality and quantity like you take care of your bank account."

The researchers acknowledged the fact that the experiments were done on mice models and thus, the findings might not be applicable to humans. The study was published in Cancer Research.