## 2 Physical Activity and the Circadian System
### Beneficial Effects of Stabilized Circadian Rhythms  
As described above, physical activity has a strong impact on circadian rhythms, Figure 1. In rodents, it has been shown that voluntary access to a running wheel helps to maintain the high-amplitude circadian rhythms of clock and clock-controlled genes in the SCN. Similar effects can also be expected to occur in humans. As a consequence of high-amplitude circadian rhythms in the SCN, their rhythmic output that controls overt rhythms and peripheral oscillators is strengthened. Proper internal and external phase relationships can be established, and a circadian disruption be abolished [109,117]. This reduces the risk of diseases. Voluntary exercise also strengthens the circadian system in elderly people. Thus, it can attenuate some age-related changes and improve their health and wellbeing. Higher levels of physical activity in older individuals correlates with a high circadian amplitude of clock gene Per3 expression [127]. Moreover, the rhythmic expression of PER3 is associated with physical fitness in older adults. [PERSON] et al. have shown that the amplitude of the circadian temperature rhythm is correlated with a higher level of physical activity in older adults [128]. The resynchronization time, e.g., following a time-zone transition, can be shortened, and the adaptation to shift work be improved by means of physical exercise. This has been shown particularly in experiments with induced jet lag [129; 130]. Moreover, physical exercise is generally useful to adjust human circadian rhythms to external time cues [131; 132] and to synchronize the whole circadian system [133]. As a consequence of stabilized circadian rhythms by means of enhanced physical activity, the cognitive performance can be improved. The SCN are involved in cognitive processes, and stable circadian rhythms are essential for cognitive abilities [134]. Disruption of the circadian sleep-wake cycle may have adverse consequences for neurobehavioral performance [135], because sleep is one of the most critical factors, which may affect cognitive abilities by improving the consolidation of information [136; 137; 138]. Moreover, processes such as learning and memory are modulated in a circadian manner. The importance of the circadian system for cognitive processes has been shown particularly in animal experiments. First, the best cognitive performance was demonstrated in studies where the temporal sequence of tests was in harmony with the 24-h rhythm [139]. Second, cognitive performance differs over the 24-h scale, or in a circadian manner [140; 141]. Third, the disruption of the circadian rhythms compromises cognitive abilities. In rats exposed to repeated phase shifts of the light-dark cycle, the learning in the Morris water-maze--particularly the long-term retention--was impaired [142]. Studies of [PERSON] et al. on arrhythmatic Djungarian hamsters revealed substantial deficits in object recognition and spatial working memory [143; 144; 145]. These hamsters were rendered arrhythmatic by light treatment [146]. In our colony of Djungarian hamsters, the arrhythmatic phenotype emerges  
Figure 1: Daytime activity facilitates robust circadian rhythms and promotes health and well-being: schematic overview. spontaneously [98]. Djungarian hamsters with a well-functioning circadian system show a pronounced daily rhythm of cognitive abilities, with the best performance during the activity period. On the other hand, animals of the arrhythmic phenotype, in which the SCN do not generate a circadian signal, not only have no rhythm in their cognitive abilities, but they also fail to perform a novel object recognition task, and show an impaired individual recognition and social memory performance [7; 141]. When the animals had access to a running wheel, part of them reestablished a circadian activity rhythm, namely those who used the wheel most intensively. In these animals, cognitive abilities were also restored [10].