Light suppresses the secretion of the neurohormone melatonin, which is an antioxidant. In any case, the International Agency for Research on Cancer has classified night work as a probable carcinogen. Millions of Americans, between 15 and 20 percent of the working population, work night shifts. That said, studies on this topic are not always consistent or conclusive, which may reflect the difficulties in accurately collecting long-term sleep data.
One study looking at sleep and colorectal cancer showed that people who had a short sleep duration before their diagnosis had an increased risk of cancer mortality, but this, like many studies, established only correlation and not causation. The Sleep Foundation's editorial team is dedicated to providing content that meets the highest standards of accuracy and objectivity. Light is a key factor in the circadian rhythm, so when not exposed to artificial light, people quickly adjust to a schedule of being awake during daylight hours and sleeping when it is dark. Although this has not been definitively proven in human studies, it provides a theoretical way that sleep and cancer may be connected.
Combining CBT-I with medication may have additional efficacy in improving sleep and quality of life. In addition, although not known to cause cancer, technology in the bedroom can cause sleep disruption, so it may help sleep if the phone is not brought to bed. A cancer patient's sleep may influence their response to cancer treatment, and a deeper understanding of the circadian rhythm may create possibilities for more effective cancer therapies. Research has found that people who sleep less than six hours a night have an increased risk of death from any cause, and a large-scale study found that people who sleep little have an increased risk of cancer.
Anyone concerned about their sleep or cancer risk should talk to their doctor to understand how this information applies to their specific situation. Some studies have found even higher numbers of sleep disorders, with nearly 70 per cent of women with breast and gynaecological cancers having symptoms of insomnia. In older adults, some research has linked reduced sleep duration to an increased likelihood of stomach cancer and found potential correlations with non-Hodgkin's lymphoma, as well as cancers of the thyroid, bladder, head and neck. Another observational study involving more than 4,000 women found an association between restless sleep and triple-negative breast cancer, an aggressive form of the disease.
In theory, then, sleeping with the lights on may interfere with normal circadian signals and create more permissive conditions for cancer to develop. More than one of these factors may contribute to sleep problems, which may vary for each individual depending on the type of cancer they have, the treatment they receive and their general health, including co-existing illnesses. In studies of people with breast cancer, treatment with cognitive behavioural therapy for insomnia (CBT-I), which seeks to reframe negative thoughts about sleep, has been shown to improve sleep and mood while boosting immune function.