One potential cause of cancer is circadian disruption, the misalignment of environmental cues (light, food intake, etc) and our endogenous circadian rhythms. Regular physical activity throughout lifetime can reduce cancer risk. Researchers asked whether this protective effect could be the most beneficial when physical activity is done in the morning.

Most studies on circadian disruption and cancer risk focused on night shift work. Recent studies suggest that exposure to light at night and late food intake may play a role in the etiology of cancer. But it remains unknown if the timing of physical activity could influence cancer risk through circadian disruption.

To address this question, researchers at Barcelona Institute for Global Health (ISGlobal), a center supported by the la Caixa Foundation, together with the Department of Epidemiology at the Medical University of Vienna, examined the effect of timing of recreational physical activity on breast and prostate cancer risk in a population based case control study. They hypothesized that the beneficial effect of the longest done physical activity in reducing cancer risk could be stronger when done in the morning. They based their hypothesis on the results of an experimental study that showed that physical activity in the afternoon and in the evening can delay melatonin production, a hormone produced mainly during the night and with well-known oncostatic properties.

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The analysis included 2,795 participants of the multicase-control (MCC-Spain) study in Spain. The researchers found that the beneficial effect of the physical activity (longest done throughout lifetime) to reduce breast and prostate cancer risk was stronger when the activity was regularly done in the morning (8-10 am). In men, the effect was similarly strong also for evening activity (7-11 pm). Results were unchanged when considering the most strenuous physical activity timing. Effects differed across chronotypes, the preference for sleeping and being active at a certain time of day. Early morning activity (8-10 am) seemed especially protective for late chronotypes, people who generally prefer to be active towards the evening.

In their paper, which was published in the International Journal of Cancer, the epidemiologists discuss how physical activity may influence human circadian rhythms and suggest possible biological mechanisms (such as alteration of melatonin and sex hormone production, nutrient metabolism etc.).

Overall the findings of this study indicate that “time of the day of physical activity is an important aspect that may potentiate the protective effect of physical activity on cancer risk,” says Manolis Kogevinas, scientific director of the Severo Ochoa Distinction at ISGlobal and coordinator of the study, in a release. “These results, if confirmed, may improve current physical activity recommendations for cancer prevention. Clear is that everyone can reduce his/her cancer risk simply by being moderately physically active for at least 150 minutes each week.”

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