Age-Related Changes in Circadian Rhythms During Adulthood

Glossary Circadian challenge: Situations in which the circadian timing system needs to adapt to changes in environmental time cues (e.g., jet lag and shift work). Circadian entrainment: Circadian rhythms are endogenous, but they are sensitive to the environment. Entrainment mechanisms exist to synchronize the endogenous circadian period of about 24 h to the environmental day–night cycle of exactly 24 h. The main environmental indicator used by the biological clock to achieve this synchronization is the light–dark cycle. Light signals reach the suprachiasmatic nuclei directly through the retinohypothalamic tract. Rods, cones, and recently discovered melanopsin-expressing intrinsically photosensitive retinal ganglion cells are needed for a proper entrainment. Main output markers of the circadian timing system in humans: Circadian rhythms are endogenous variations in psychological or physiological functions over a period of about 24 h ('circa diem,' about one day). In human chronobiology, two rhythms are most often used as markers of the circadian timing system: the circadian rhythm of body temperature and the circadian rhythm of the production of pineal melatonin. Markers of the phase of the circadian timing system in humans: To estimate the timing of the internal biological clock (the circadian phase), the most often used markers are the time of the minimum of the rectal temperature rhythm, which occurs usually about 2 h before habitual wake time; and the time of onset of the daily episode of melatonin production, which occurs about 2 h before habitual bedtime. In addition to period and phase, circadian rhythms are also characterized by their amplitude, which is half the difference between the minimum and the maximum values of the function over the 24-h period. Nonvisual responses to light: Functions other than vision (i.e., not related to image formation) regulated at least partly by light such as circadian entrainment, pupil constriction, melatonin suppression, alertness, and vigilance levels.