The paper describes some features of modern and future interplanetary space missions which may influence the stability of the circadian system of the human body. It presents requirements for biorhythmological selection of cosmonauts. The paper discusses the biorhythmological types of humans: biorhythmologically labile, biorhythmologically inert and intermediate. The time schedule specified by the flight program dictates the selection of the cosmonauts of a certain type for the given mission. The paper suggests methods for the biorhythmological selection of cosmonauts: 1) on the basis of adaptation to the concrete type of "space day" in the flight; 2) on the basis of the biorhythmological type; 3) on the basis of the length of the natural circadian period; 4) on the basis of typological features of the nervous system.