Are trajectories of insomnia symptoms associated with clinically significant depressive symptoms or vice versa?...

To examine the association between different insomnia symptom trajectories and subsequent depressive symptoms, as well as the reverse.

This was a secondary analysis of a population-based study on the natural course of insomnia in Canada, and 3030 participants (18-94 years) with valid data were included. Two separate analyses were conducted for the distinct objectives of the current study, which required different samples. Participants who exhibited the designated outcomes at baseline were excluded from each analysis. Growth mixture modeling was used to identify insomnia and depressive symptom trajectories over five years, measured by Insomnia Severity Index (ISI) and Beck Depression Inventory-II (BDI-II), respectively. Cases with clinically significant depressive symptoms at follow-ups were defined as reporting at least moderate depressive symptoms (BDI-II ≥ 20) and one core symptom of depression, while insomnia disorder at follow-ups was identified according to a validated algorithm using a combination of criteria from DSM-4 and ICSD-3 and self-reported use of sleep medication.

Five insomnia symptom trajectories were identified over five years: stable good sleepers (24 %), stable low insomnia severity (35 %), gradual improvements in symptoms (6 %), persistent insomnia symptoms (31 %), and progressive worsening of symptoms (5 %). Compared with good sleepers, the other insomnia trajectory groups all had higher risks of clinically significant depressive symptoms at follow-ups, especially those with progressive symptoms (harzad ratio [HR]: 19.77 [10.30 to 37.93]). In a parallel analysis, four depression trajectories emerged: noncases (49 %), stable low depressive severity (38 %), moderate symptoms improving over time (7 %), and progressive worsening of symptoms (6 %). Compared with noncases, the other depression trajectory groups all showed elevated risks of insomnia disorder, especially those with progressive worsening of symptoms (HR: 6.85 [5.26 to 8.93]).

Our findings support a close relationship between insomnia and depressive symptoms, especially when both symptoms become progressive or persistent.