Ageing of the circadian system. From monitoring to chronoenhancement

The circadian system (CS) organizes the temporal order of all living beings. Its general structure is very similar among species and consists of receptors of the temporal information (inputs), a central pacemaker along with several peripheral clocks that depend on it (machinery), and a set of overt rhythms driven by the central clock (outputs). The CS ages like any other structure of the organism, this process being characterized by a poorer reception of the temporal information, a general impairment of the central pacemaker and a phase advance, fragmentation and dampening of the overt rhythms. In order to assess the functioning of the CS, some overt rhythms have been selected as markers since they mirror the activity of the central pacemaker. Some of the most used marker rhythms are those of melatonin and cortisol secretion, rest-activity and sleep-wake patterns, and core body and distal skin temperature. Nevertheless, these rhythms can be masked by external variables and thus, to simultaneously record several marker rhythms is recommended. As the CS ages, the ability of an organism to adjust the internal temporal order of physiological, biochemical and behavioural circadian rhythms to the environmental cycles is compromised and chronodisruption can appear, which is related with several diseases. Fortunately, there are some strategies that one person can follow in order to enhance the functioning of the CS: to increase the contrast between day and night (i.e. to exposure to bright days and dark nights), to have melatonin (if needed), to do regular exercise, to improve sleep and meal schedules or to increase social contacts.