Longitudinal effects of nocturnal insomnia symptom subtypes and nonrestorative sleep on the incidence of depression among community-dwelling older adults: results from the Health and Retirement Study

The relationships between different insomnia symptom subtypes and the onset of depression among older adults are inconsistent. It may be that each subtype has a distinct temporal effect on depression not easily captured by the different follow-up intervals used in past studies. We systemically investigated the temporal effects by examining the links between subtypes and the onset of depression at different follow-up intervals among community-dwelling older adults. We used the 2006 wave of the Health and Retirement Study as baseline (n = 9151). The outcome was the onset of depression at 2-year (2008 wave), 4-year (2010 wave), and 6-year (2012 wave) follow-ups. The independent variables were difficulty with falling asleep (initial insomnia), waking up during the night (middle insomnia), waking up too early and being unable to fall asleep again (late insomnia), and nonrestorative sleep at baseline. Factors known to be related to depression among older adults were included as covariates. Our findings showed that each insomnia symptom subtype had distinct temporal effects on the onset of depression. It appeared that the effects of initial insomnia may take longer to emerge than indicated in previous studies. Middle insomnia and late insomnia had weak relationships with depression. Nonrestorative sleep predicted the onset of depression at every follow-up period. We found that documenting the temporal effects of insomnia symptom subtypes helps both to classify individuals' insomnia symptoms and predict the onset of depression. We recommend taking temporal effects of insomnia symptom subtypes into account in future investigations and clinical practice.