The time of day we exercise could affect the risk of cancer, according to a new controlled study conducted with almost 3,000 Spanish people.

Studies have shown that one potential cause of cancer is circadian disruption, the misalignment of environmental cues—such as light and when you eat—and the internal, biological circadian rhythms.

It is established that regular physical activity throughout your lifetime can reduce cancer risk, but this protective effect could be the most beneficial when physical activity is done in the morning, according to a recent study coordinated by the Barcelona Institute for Global Health (ISGlobal), together with the Department of Epidemiology at the Medical University of Vienna.

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 cancer risk. However, to date it remained unknown if the timing of physical activity could influence cancer risk due to circadian disruption.

To address this question, the researchers 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 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 which showed that physical activity in the afternoon and in the evening can slow melatonin production, a hormone produced mainly during the night and with well-known anti-cancer properties.

"The timing of physical activity obviously has an effect upon the rhythm of sex hormones and melatonin – as well as on food metabolism," said study co-ordinator Dr Manolis Kogevinas, of the Barcelona Institute for Global Health. "That might explain our results."

The analysis included 2,795 participants of the multicase-control MCC study in Spain. The researchers found that the beneficial effect of the physical activity to reduce breast and prostate cancer risk was stronger when the activity was regularly done in the morning (8-10 am), with "results unchanged when considering the most strenuous physical activity timing."

Effects differed when considering individual differences in activity and alertness in the morning and evening. Early morning activity (8-10 am) seemed especially protective for night owls, people who generally prefer to be active towards the evening, and whose melatonin production might be slowed.

In their paper, which was published in the International Journal of Cancer, the epidemiologists discuss how physical activity may influence human circadian rhythms and biological mechanisms, like altering of melatonin and sex hormone production, nutrient metabolis 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", commented Manolis Kogevinas, Scientific Director of the Severo Ochoa Distinction at ISGlobal and coordinator of the study.

"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", he added.

Source: Barcelona Institute for Global Health (ISGlobal)