_applied sciences_  
_Review_  
**The Impact of Physical Activity on the Circadian System: Benefits for Health, Performance and Wellbeing**  
**[PERSON] 1,* and [PERSON] 2,3,4(c)**  
**Key words:** circadian rhythms are an inherent property of all living systems and are generated by an internal clock. In mammals, this is localized in the suprachiasmatic nuclei (SCN) of the hypothalamus [1]. The SCN as central pacemakers are involved in the rhythmic regulation of sleep-wake behavior [2], locomotor activity [3], metabolism [4], body temperature [5], and other body functions. Circadian rhythms are an essential part of the external and internal temporal order. They enable organisms to be synchronized with their periodic environment, to be prepared for rather than react to periodic changes and coordinate physiological and behavioral processes. This guarantees an optimal functioning, and any disturbances, or so-called circadian disruptions, may have adverse consequences for health, physical and mental performance, wellbeing, and longevity [6; 7; 8; 9; 10]. Such disruptions do occur during aging [11; 12; 13; 14], following time-zone transitions [15; 16; 17], in shift-workers [18; 19], and in subjects suffering from different diseases [20; 21; 22; 23]. However, in the latter case, it is not clear if circadian disruptions are a consequence of the disease or a cause. Therefore, it is highly important to find approaches to synchronize circadian rhythms and, thus, stabilize the entire circadian system. Melatonin, as an effective chronobiotic, is often used [24]. However, the physiological effects of melatonin depend critically on dosing and timing. For chronobiotic purposes, physiologic doses (3 mg or less) are useful, while higher doses (3-10 mg or more) exert antioxidant and immunoregulatory effects [25; 26]. Optimal dosing may be different between individuals. Even more important could be a personal adjustment of timing, since physiological responses to melatonin follow phase response curves [27]. When non-physiological doses have to be used, one must consider adverse side effects. Accordingly, non-pharmacological approaches may be preferred, except, for example, in very old people where the use of melatonin might be unavoidable [28; 29]. The daily light-dark cycle is the main zeitgeber [30; 31]. It synchronizes circadian rhythms with the 24-h environment. In our modern society, however, most people are exposed to only low levels of light in the daytime. On the contrary, the light levels in the evening or at night are often too high. Both have adverse consequences for the synchronization of the circadian rhythms with the 24-h environment and multiple negative effects on physiology, sleep, physical and mental performance, and well-being [19; 32; 33; 34; 35]. Accordingly, circadian light hygiene and a proper 24-h light regimen in the offices and homes become increasingly important [36; 37; 38]. Food can also serve as a synchronizer for the circadian system. Timed feeding may reset the SCN phase via a reinforcement of clock gene expression, entrain peripheral clocks, and facilitate intrinsic synchronization [39; 40; 41; 42]. Thus, it can be used to strengthen the circadian system, which in turn promotes healthy aging and longevity [21; 43; 44]. Robust circadian clocks can also be facilitated by lifestyle, regular physical activity, and timed exercise. Aged people often adopt, although rather intuitively, a regular lifestyle [45; 46]. However, a problem of our modern society is a general low level of physical activity [47]. This has adverse consequences for the cardiovascular system and the energy metabolism [48]. Moreover, direct effects on homeostatic physiological mechanisms and disturbances of the circadian system have a considerable impact. Scheduled physical activity is an efficient strategy to maintain circadian rhythms and contribute to healthy ageing [12; 13; 49]. It upholds feedback coupling with the central brain clock and helps to preserve synchronized circadian rhythms [10; 50].