Administering radiation treatments in the evening rather than the morning could help persons with cancer keep more of their hair, a mouse study indicates.

After identifying two key sites of peripheral circadian clock activity specific to the regeneration of hair follicles, a research team showed that peripheral circadian clock at one of those sites, epithelial matrix cells, generated prominent mitotic rhythm that caused hair to grow faster in the morning than in the evening.

“Because cells are the most susceptible to DNA damage during mitosis, this cycle leads to a remarkable time-of-day-dependent sensitivity of growing hair follicles to genotoxic stress,” explained Maksim V. Plikus, Assistant Professor of Developmental and Cell Biology at University of California, Irvine, and fellow investigators in Proceedings of the National Academy of Sciences of the United States of America (2013;110[23]:E2106-E2115).

The group found that mice lost 85% of their hair if they received radiotherapy in the morning, but lost only 17% of their hair when they underwent evening treatments.

Studies will be needed to determine whether these findings will directly translate to human cancer therapy. However, the results can help to shape strategies for minimizing or maximizing cytotoxicity of radiation therapies by timing such treatments to when a given organ is most or least active in its circadian cycle.