The effects of COVID-19 lockdown and reopening on rest-activity rhythms in Singaporean working adults: A longitudinal age group comparison study.

Study objectives: COVID-19 mobility restrictions disrupted daily rhythms worldwide, but how this rhythm disruption differs across age groups is unclear. We examined the course of age-related differences in trajectories of rest-activity rhythm during the COVID-19 pandemic lockdown and reopening in Singapore. We also evaluated the association of these patterns with mental well-being.

Methods: 24-hour step count data (Fitbit) were obtained from 617 younger (age range: 21-40) and 602 older adults (age range: 55-70) from January 2020 (baseline) through lockdown (April 2020) and reopening periods until August 2021. Nonparametric rest-activity rhythm metrics: interdaily stability, intradaily variability and most active 10-hour period (M10) were computed. Longitudinal changes in rest-activity rhythm, age-related differences in changes, and the associations between mental well-being and these changes were assessed using nonlinear latent-growth models.

Results: In younger adults, mobility restrictions during lockdown caused significant decline in interdaily stability and M10, alongside significant increase in intradaily variability. However, in older adults, changes were confined to increased intradaily variability and decreased M10. Older adults also showed less change in intradaily variability and M10 compared to younger adults. Gradual recovery of rest-activity rhythm metrics during reopening was observed, with interdaily stability and M10 remaining lower after 15months post-lockdown. In younger but not older adults, a larger decline in interdaily stability was associated with poorer mental well-being 15months post-lockdown.

Conclusion: Younger adults appear more vulnerable than older adults to mobility restrictions as reflected in their rest-activity rhythm metrics. A significant disruption of daily routine may have long-lasting effects on younger adults' mental well-being.

Statement of significance: Although stringent mobility restrictions imposed to curb the spread of COVID-19 were imposed primarily to protect older adults, we found that younger adults were more vulnerable to rhythm disruption arising from mobility restrictions. Disrupted rhythm stability was associated with poorer mental well-being 15months after the lockdown ended in younger but not older adults. These asymmetric long-term effects on mental health on younger relative to older adults should be kept in mind when planning for large-scale catastrophes linked to mobility restrictions.