Prospective associations of insomnia markers and symptoms with depression

Abstract

Whether insomnia, a known correlate of depression, predicts depression longitudinally warrants elucidation. The authors examined 555 Wisconsin Sleep Cohort Study participants aged 33-71 years without baseline depression or antidepressant use who completed baseline and follow-up overnight polysomnography and had complete questionnaire-based data on insomnia and depression for 1998-2006. Using Poisson regression, they estimated relative risks for depression (Zung scale score > or =50) at 4-year (average) follow-up according to baseline insomnia symptoms and polysomnographic markers. Twenty-six participants (4.7%) developed depression by follow-up. Having 3-4 insomnia symptoms versus none predicted depression risk (age-, sex-, and comorbidity-adjusted relative risk (RR) = 3.2, 95% confidence interval: 1.1, 9.6). After multiple adjustments, frequent difficulty falling asleep (RR = 5.3, 95% confidence interval: 1.1, 27.9) and polysomnographically assessed (upper or lower quartiles) sleep latency, continuity, and duration (RRs = 2.2-4.7; P's < or = 0.05) predicted depression. Graded trends (P-trend < or = 0.05) were observed with increasing number of symptoms, difficulty falling asleep, and difficulty returning to sleep. Given the small number of events using Zung > or =50 (depression cutpoint), a limitation that may bias multivariable estimates, continuous depression scores were analyzed; mean values were largely consistent with dichotomous findings. Insomnia symptoms or markers increased depression risk 2.2- to 5.3-fold. These results support prior findings based on self-reported insomnia and may extend similar conclusions to objective markers. Heightened recognition and treatment of insomnia may prevent subsequent depression.

Figure 1.

Incidence of depression symptoms (%) at 4-year follow-up (2002–2006), obtained using different classifications of depression, by number of insomnia symptoms at baseline (1998–2002) in Wisconsin Sleep Cohort Study participants (n = 542–685). White columns, no symptoms; columns, with squares, 1 symptom; striped columns, 2 symptoms; black columns, 3 or 4 symptoms. “Modified Zung score” refers to the Zung depression scale (38) rescored after exclusion of 2 items on insomnia and tiredness. P-trend values for the relation of number of insomnia symptoms to subsequent depression: for Zung score ≥50, P-trend = 0.006; for Zung score ≥50 excluding antidepressant users at follow-up, P-trend = 0.02; for modified Zung score ≥50, P-trend = 0.02; and for modified Zung score ≥50 excluding antidepressant users at follow-up, P-trend = 0.06. Bars, standard error.

Figure 2.

Incidence of depression symptoms (%) at 4-year follow-up (2002–2006) by frequency of insomnia symptoms at baseline (1998–2002) in Wisconsin Sleep Cohort Study participants (n = 555). White columns, never or rarely experienced symptom; striped columns, sometimes experienced symptom; black columns, often or almost always experienced symptom. “am,” morning; “pm,” night. Bars, standard error.