Chronobiology is the study of those biological phenomena that change periodically with time. Botanicals are a good example of yearly periodicity, with blossoming and fructification on a yearly period. In humans, the best-known phenomena are the daily cycle of wake and sleep and the monthly cycle of female fertility.
Far from being an academic curiosity, circadian (from the Latin circa diem, around the day) rhythms play relevant medical roles. Let us consider one example. The treatment of some cancers requires drugs that have the undesirable side effect to be toxic for the lymphocytes. The number of lymphocytes per unit volume in human blood changes according to a circadian rhythm.
It is known that a drug, at a certain concentration, “kills” a certain fraction (a fraction, NOT a defined number) of target cells; so, if the drug is administered when the lymphocytes are present in larger number, the number of lymphocytes surviving the treatment will be larger, and the consequences of the drug toxicity will be more tolerable. In a comparative study of treatment of ovary cancers with cis-platinum and doxorubicin, the survival after five years of the patients who received chemotherapy at the time of the day corresponding to the lymphocyte maximum was 75%, whereas no patients survived more than three years when the drugs were administered at random times; i.e., without considering the circadian rhythm of the lymphocytes.1