Night shift work appears to be a risk factor for breast cancer in large part because of its disruptive effects on circadian rhythms. Circadian synchronization depends on an internal clock that is synchronized to light-dark cycles. Circadian rhythms are popularly thought of as being driven solely by the master clock in the brain, which receives information directly concerning light-dark conditions through the eyes. However, individual cells in other organs also have clocks.

Circadian rhythm disruption is not a misalignment of the master clock in the brain and the earth's 24-hour light-dark cycle (although this misalignment may contribute to it). Rather, it refers to when the master clock in the brain becomes out of sync with local cellular clocks in other organs, including the breasts. Although the process is not well understood, repeated desynchronization appears to result in degradation of the circadian cell cycle, which in turn facilitates uncontrolled growth. Now a new study has reported that women with stage IV breast cancer who are unable to sleep at their preferred bedtimes have shorter disease-free intervals than those whose sleep patterns conform to their chronotype ("morning person" or "night owl").

Light at night and breast cancer

Light in the bedroom at night is associated with increased risk of breast cancer. Habitually sleeping in the presence of artificial light appears to increase breast cancer risk by inhibiting melatonin production. Melatonin is a hormone that communicates information concerning environmental light conditions to various parts of the body and plays a vital role in the synchronization of circadian rhythms. Melatonin is synthesized and secreted by the pineal gland in the brain. Blue light wavelengths appear to have a far greater suppressive effect on melatonin production than red wavelengths. Women can maintain their melatonin levels by getting enough sleep, avoiding night shift work and light exposure at night, as well as consuming melatonin-rich foods.

Latest research finds inability to sleep at regular desired time can shorten survival

The study referenced at the beginning of this news story was designed to investigate whether disruption of circadian rhythm might be linked to breast cancer progression. In other words, for example, is progression accelerated if a night person often goes to sleep early? There are differences in circadian physiology associated with chronotype. This is the first study to examine the effect of chronotype and alignment of bedtime with chronotype on recurrence.

The study included 85 women with Stage IV breast cancer. To conduct the study, the authors first determined each woman's chronotype and alignment of actual bedtime with preferred chronotype using the Morningness-Eveningness Scale (MEQ) and a sleep-wake log. Misalignment of bedtime on a daily basis is an indication of circadian disruption. Median disease-free interval was 81.9 months for participants with aligned bedtimes (i.e., who normally went to bed at their preferred bedtimes) (72 women) and 46.9 months for women with misaligned bedtimes (who went to bed later or earlier than their preferred bedtimes) (13 women). The results held when the authors controlled for other significant predictors of disease-free interval. The authors conclude that a misalignment of bedtime is associated with faster breast cancer progression, as measured by disease-free interval. A larger prospective study is needed, according to the authors, to overcome the limitations of small sample size and study design, and to investigate their causal relationships and underlying mechanisms.