The aging clock: circadian rhythms and later life

Circadian rhythm (SCN firing). The graph at the top illustrates firing in the suprachiasmatic nucleus (SCN) in younger (blue) vs older adults (red). In mammals, the SCN is the master circadian clock.


Observations of many species have revealed a variety of developmental changes in circadian rhythms of overt behaviors and physiology. In reviewing this literature, it is worthwhile to note some aspects of research design that may influence the conclusions that can be drawn. For example, the vast majority of research in this area is cross-sectional; as such, differences in dependent variables between older and younger individuals that are attributed to age could potentially reflect cohort effects or other confounders. This is a legitimate criticism; however, we believe it is mitigated by the number of studies that have identified similar patterns of results using different samples and age cohorts.

Circadian rhythm (waking). Shifts in preference for morningness versus eveningness, or chronotype, are seen in many species.

Shifts in preference for morningness versus eveningness, or chronotype, and in sleep cycles are among the most consistently observed age-associated circadian changes in many species. Broms and colleagues tracked chronotype longitudinally in 567 adult men in Finland over 23 years (mean age of 56 years at study entry), and found a shift in the distribution toward a “mostly morning” type over years of study. Retrospective self-comparison studies in older adult participants (>60 years) also indicate a tendency to become a “morning person” with increasing age. Taken together, this shift in chronotype appears to be a reliable developmental pattern.

The preference for morningness in older adulthood is expressed in other aspects of behavior, such as cognitive skill performance. The circadian profile of cognitive performance interacts with age, such that older adults who are tested on recognition memory tasks in the early morning perform as well as younger adults, but significantly worse when tested later in the afternoon.

FULL TEXT: J Clinical Investigation