A few years ago, two studies found that people with sleep apnea have a much higher rate of cancer than people without it, even controlling for several cancer-related variables. In one study, the increase in risk was five-fold. These studies raised several questions: 1. Were the associations due to chance? 2. If real, did the associations reflect cause and effect? Surely people with sleep apnea are different in several ways from people without it. 3. If the associations did reflect cause and effect, which of the many effects of sleep apnea was/were responsible?
A new study found that rats woken up frequently got more cancer. This is a correct prediction from the idea that bad sleep increases cancer, increasing the plausibility of that idea.
[The study] used mice, housed in small groups. During the day—when mice normally sleep—a quiet, motorized brush moved through half of the cages every two minutes, forcing those mice to wake up and then go back to sleep. The rest of the mice were not disturbed. After seven days in this setting, both groups of mice were injected with cells from one of two tumor types (TC-1 or 3LLC). All mice developed palpable tumors within 9 to 12 days. Four weeks after inoculation the researchers evaluated the tumors. Tumors from mice with fragmented sleep were twice as large, for both tumor types, as those from mice that had slept normally. A follow-up experiment found that when tumor cells were implanted in the thigh muscle, which should help contain growth, the tumors were much more aggressive and invaded surrounding tissues in mice with disrupted sleep.
Great Sleep! Reduced Cancer! is a whole book (98 pp) about the connection between sleep and cancer.
Epidemiologists haven’t yet figured out that they should always measure sleep quality, just as they always measure cigarette smoking and body weight, but at least interest is growing. Both short and long sleep are associated with a higher risk of heart disease.