## 2 Physical Activity and the Circadian System  
### The Diagnostic Value of Circadian Activity Rhythms  
In humans, activity can be monitored easily over many days and at short sampling intervals by means of non-invasive devices worn on the wrist of the non-dominant hand [51]. This method is widely used in field research and in clinical settings, as it is simple, inexpensive, and rather accurate [52; 53; 54]. In laboratory animals, motor activity can be monitored by transmitters that are implanted into the peritoneal cavity and allowing the simultaneous recording of core body temperature [55]. Moreover, most reliable and less expensive are passive infrared motion detectors that are mounted above the cage roof [56]. Both methods record general activity, not just locomotion.  
The wide use of actinetry monitoring techniques makes circadian activity rhythm assessment a useful tool to survey circadian rhythm robustness [57; 58]. Moreover, rhythm characteristics can be used as indicators of physical and mental fitness and the health status of human beings. [PERSON] and co-authors [59] usedtigraphy to characterize the activity level and the daily activity rhythm of patients following hip and knee joint replacement and designed specific, personalized rehabilitation programs.  
The inter-daily rhythm stability and the median level of daytime activity were found to be negatively correlated with cognitive decline and depression in elderly women [53]. Other authors compared the amounts of activity when in bed or out of bed. This can be a preferable option when the 24-h pattern is non-sinusoidal. The estimated dichotomy indices were decreased in diseased individuals, particularly in those suffering from colorectal cancer [54; 60]. Together with the mean activity level and autocorrelation coefficients, they are an objective indicator of physical welfare and an appropriate prognostic factor for cancer patients' survival and tumour response [61; 62; 63]. According to [PERSON] et al., characteristics of the rest-activity rhythm, especially the inter-daily stability and intra-daily variability, can be used as biomarkers of cardiometabolic health [64]. Irregular sleep duration and timing were recognized as novel cardiovascular risk factors, independent of other traditional factors and sleep quantity or quality [65]. An increased fragmentation of the 24-h activity rhythm is also positively associated with symptoms of food addiction [66]. Differences in the period length of the free-running activity rhythm were found in spontaneous hypertensive rats as compared to healthy controls [23]. Remarkably, changes occurred in the pre-hypertensive state already and not after the surgical induction of hypertension. In glaucoma patients, the sleep-wake rhythm was found to be compromised [67].  
In laboratory animals, the onset of their main activity period is a convenient phase marker. It characterizes the strength of photic entrainment and undergoes predictable changes, for example, following a phase shift of the light-dark cycle [55; 68] or during aging [69; 70]. In old mice, a phase advance and a decreased phase stability of the activity onset were observed together with an increased inter-daily rhythm variability. These changes were found to happen earlier than the decrease in total activity per day and can be used to characterize the biological age [71; 72]. Interestingly, similar changes were observed for the sleep-wake rhythm of aged people, particularly those suffering from Alzheimer's disease (AD) and Parkinson's disease [53; 73; 74; 75]. In mild-to-moderate AD, increased fragmentation of the activity pattern predicted cognitive decline that was independent of age, sex, educational level, and season [76]. Interestingly, this work also reported an increased circadian amplitude, illustrating that rhythm stability, amplitude, and phase should be considered altogether to examine the distinctive types of circadian disruption, e.g., \"extra-circadian dissemination\" [57].