Postural sway variability in young adults presents higher complexity during morning compared to evening hours while in older adults remains the same.

Human movement variability reflects the adaptive capacity of the nervous system, yet how it is influenced by aging and circadian rhythms remain unclear. Therefore, the purpose of this study was to investigate postural sway variability as a function of aging and time of day. Nineteen young and nineteen older adults completed one 60-s quite stance trial with eyes open while standing on a force platform, at 12 p.m. and 12 a.m. Postural sway variability was evaluated regarding both its magnitude (total travel distance and interquartile range) and the complexity (a exponent using Detrended Fluctuation Analysis) of its temporal structure using the center of pressure time series. A two-way ANOVA (2 age groups × 2 times of day) was used. Correlation analysis was also performed to further investigate the relationship between circadian regulation and postural sway complexity. Complexity was higher for the young compared to the older group independently of the time of day. Furthermore, young adults presented higher values during the morning as compared to evening, while older adults did not reveal significant differences within the day. Finally, a strong correlation was found but only for young adults. In general, our results suggested that complexity of postural sway variability is affected both by age and time of day. Aging impacts postural control by reducing the complexity of sway variability and diminishing its sensitivity to circadian influences. Future work will address the effect of chronotype, sleep, and arousal levels on these novel findings and assess their impact on overall health.