Effects of Circadian Rhythm on Physical and physiological Performance of Military forces- Narrative Review

The 2017 Nobel Prize for medicine was awarded the biological clock Scientist, which shows the importance of this phenomenon in the life of living organisms. The circadian Rhythm (CR) through the created internal “clock” is responsible for regulating the daily performance of different organs of the body. The central body clock is the key factor to creating and maintaining this CR. External optical cues and non-optical environmental indications (Zeitgeber), including optical stimuli, eating time, environmental temperature and exercise, are able to influence and regulate the CR. During military activities, there is always an innate conflict between the body's internal clock and military working programs. There are also many reports about sleep disorders and malnutrition in military forces. The long-term desynchronization of physiological processes with CR may lead to impaired physical, physiological, metabolic, mental and cognitive functions of military personnel.In addition to the impact of CR on physiological systems, the priority of the special time of day or night for activities is also another important factor to be considered. The preferential difference of activity time (known as ‘chronotypes’) is referred to in different physiological rhythm, including physical performance and response to sports activity. Better performance and muscular fatigue during exercise during the evening can be justified through higher central temperatures, catecholamine concentrations, lactate levels, and biological markers of muscle damage. However, considering the interaction between the time of day (TOD) and the impact of physical activity on the response of biochemical parameters, the CR of these parameters in the military force is not fully defined during physical activity. Designing suitable sport interventional strategies to improve short/long-term military health is a key issue in promoting metabolic health. Putatively, correct timing of exercise bouts might ameliorate some deleterious results of acute and chronic shift work.