Background: Melatonin is a biomarker of the central circadian clock and its chronobiotic
actions entraining circadian rhythms to the light-dark cycle are well known. Reduction in melatonin
levels and altered circadian rhythms have been associated with a high risk of breast cancer. Melatonin
has also been shown to display anti-proliferative effects on breast cancer growth and proliferation.
Evaluation of melatonin circadian rhythm alterations in patients bearing breast cancer may have interesting
prognostic and therapeutic applications.
Objective: To review studies evaluating the circadian rhythm of melatonin in breast cancer patients.
The effects of surgery and chemotherapy on melatonin secretion were also reviewed.
Methods: Electronic databases, including PubMed/MEDLINE and Scopus, were searched from their
inception to May 2020, using the keywords "Melatonin", "Circadian rhythm" and "Breast cancer".
Results: Patients with breast cancer maintain a circadian rhythm of melatonin secretion with relatively
high levels during the night and low levels during the day, however, a reduction in nocturnal melatonin
peak and decreased amplitude of melatonin circadian rhythms in these patients have also been reported.
Melatonin levels can influence estrogen receptor concentrations in hormone-dependent estrogen-
positive breast cancer. Chemotherapy alters melatonin levels and breast surgery tends to alter
melatonin secretion at first-day post-operation. Melatonin levels correlate with clinical and psychological
symptoms of breast cancer, such as sleep quality and depression severity.
Conclusion: Circadian rhythm and the concentration of melatonin in the blood are altered in patients
with breast cancers, and it can modify not only the sleep-wake cycle and, thus, patients’ quality of life
but due to melatonin’s antioxidant effects, the effect of therapies can be modulated. Due to the heterogonous
protocols used to assess melatonin and variable environmental factors during sampling, further
studies need to control, such variables in order to tailor clinical trials based on melatonin rhythm adjustment
and/or supplementation in breast cancer patients.