## 2 Physical Activity and the Circadian System
### Effects of Motor Activity on the Circadian System  
The above-mentioned effects of physical activity are at last in part mediated via the circadian system. It has been well known for many years that motor activity has a considerable impact on circadian rhythms, via a feedback effect on the central pacemaker, the SCN. A correlation between activity level and the period length of the rhythm generated in the SCN was found [97; 56; 98]. Intensive running wheel use also affects phase responses [99; 100; 101]. Bouts of physical activity may cause phase changes--so-called non-photic phase responses. Depending on the circadian time, activity bouts may cause a phase advance or a phase delay. [PERSON] first published a non-photic phase response curve [102]. In Golden hamsters for example, a bout of motor activity in the middle of the light time, i.e., during the subjective day, causes a phase advance. The animals' next activity period starts earlier, i.e., the animals \"wake up\" and \"go to sleep\" earlier. Similar effects were described for humans [103]. One hour of moderate treadmill exercise for three consecutive days caused significant phase changes of the urinary aMT6s rhythm, the metabolic end-product of melatonin. Motor activity and behavioral states may directly influence the neurophysiological properties of the SCN and affect clock genes expression [104; 105; 106; 107]. This may be the causal reason for the above-mentioned changes in photic phase responses and free-running periods. [PERSON] and co-authors [101] argued that the circadian rhythms of Djungarian hamsters can be stabilized by enhanced motor activity. Indeed, [PERSON] and coworkers demonstrated that wheel running rendered the synchronization of the circadian body temperature rhythm in the golden hamsters [108]. This hypothesis was further supported by studies on aging mice. When animals had access to running wheels, the activity onsets, which were highly variable and phase advanced, became stronger coupled to \"lights-off\" [14; 109]. In Djungarian hamsters with a delayed activity onset, photic entrainment was stabilized by means of elevated motor activity [98; 110]. Like many other rodent species, Djungarian hamsters use running wheels voluntarily, regularly, and intensively [111; 112]. Some of them eventually developed a wild-type activity pattern with its onset being tightly coupled to \"lights-off\"; however, this phenomenon could not be explained by the observed changes of endogenous period lengths and photic phase responses. Obviously, mechanisms downstream from the suprachiasmatic nuclei are involved. Circadian rhythms are reinstated, and cognitive performance is improved in arrhythmic (AR) Djungarian hamsters when they intensively use running wheels [10]. The underlying mechanisms of the AR phenomenon are not yet known. The loss of synchronization between single SCN neurons, similar to what was found in aged mice, is a putative factor [113]. In SCN slices of AR Djungarian hamsters a broad range of firing frequencies in the electrical activity was found [114]. The authors argued that this indicates a loss of synchrony among SCN neurons. Our study showed that resynchronization and a proper phase relationship between SCN neurons can be achieved by motor activity. Regardless, a more robust output signal downstream from the SCN should foster consistent overt circadian rhythms such as activity. The observed phenomenon might also be caused by another mechanism. In vivo studies demonstrated that behavioral activity may suppress the firing rate in the SCN of freely moving rodents, e.g., rats and hamsters [106,115], and mice [116]. Since behavioral activity can modulate the SCN firing rate, it may also boost circadian rhythms when activity occurs at the specific time (at night in the case of nocturnal species). These findings are supported by a study which showed that scheduled daily activity can improve behavioral circadian rhythms in mice with compromised SCN function [117].