periodic alternation of the state of tension and relaxation in the mental activity of a person. A distinction is made between external biorhythms – their manifestation is associated with the cyclical nature of solar activity (11.5 years), the change of seasons, days, etc., and internal biorhythms – determining the state of activity and relaxation of physical and mental activity. Of particular importance is the daily biorhythm of mental activity, which affects the periods of the greatest working capacity and fatigue: the maximum activity is observed in the morning (8 – 12 hours), the minimum – in the middle of the day (12 – 16 hours), the second maximum – in the evening (16 – 2 hours) and a particularly pronounced minimum – at night (2 – 8 hours). The alternation of the maximum-minimum of physical activity and mental activity during the day corresponds to the scheme: tension – relaxation – tension – relaxation and is consistent with the daily change in the internal biorhythms of mental activity, manifested in the activity of the sympathetic-adrenal system. An increase in activity in the morning and evening corresponds to an increase in the release of hormones: adrenaline and noradrenaline. Such a rigid dependence of the active and inactive components of mental activity on the internal biochemical processes occurring in the body requires a clear correspondence between the internal biorhythms and the external organization of life. If the correspondence is violated, then the frequent consequences of this are: sleep disorders, neuroses, diseases of the cardiovascular system. Therefore, the rehabilitation process must necessarily take into account the structure of biorhythms of the rehabilitator’s mental activity.