Depression scores absent sleep items on both the HRSD and the BDI-II significantly improved across time in the uncontrolled IPT group (both p < .001), where the effect sizes for IPT were g = 0.92 and g = 1.32, respectively. No significant improvements are noted for insomnia, where the effect size was g = 0.49. Results are further detailed in .
Effect of Treatment on Depression and Insomnia.
HRSD and the BDI-II depression scores absent sleep items significantly improved in the RCT IPT group with χ2(3, N = 36) = 17.56, p < .001 and χ2(3, N = 36) = 13.94, p < .001, and effect sizes of g = 1.10 and g = 0.85, respectively. Depression scores, however, also improved in the TAU condition, albeit these improvements were more modest (see ). Across-treatment effect sizes for TAU were g = 0.76 for the HRSD and g = 0.64 for the BDI-II. In contrast, after controlling for improvement on the HRSD as an additional covariate, insomnia scores did not significantly improve in either the IPT or TAU condition with χ2(3, N = 36) = 6.94, ns and χ2(3, N = 32) = 4.60, ns, although a trend was evident at the level of p = .07 in the IPT condition. Across-treatment effect sizes for insomnia were smaller than those observed for depression
In group x time contrasts, participants in the IPT condition improved relative to the TAU condition on the HRSD with χ2(3, N = 68) = 5.00, p < .05 and BDI-II with χ2 (3, N = 68) = 4.25, p < .05. Effect sizes for IPT compared to TAU on depression were small to moderate (). There was a trend for greater improvement in insomnia in the IPT condition with χ2(3, N = 68) = 3.00, p = .07). Effect sizes were small in these between-treatment contrasts for all three variables ().
No significant relationship was observed between number of treatment sessions and the magnitude of improvement on the HRSD, BDI-II or the insomnia score. Adding the covariate of completing ≥ 6 treatment sessions to the RCT between-group analyses did slightly alter the size of observed group differences from the primary analyses. The IPT condition continued to have greater improvements than the TAU condition on the HRSD (p <.05), while findings on the BDI-II moved from significance to a strong trend (p = .054) and insomnia findings moved from a trend to significance (p < .05); all effect sizes remained small with g = 0.29, 0.26 and 0.30, respectively.
In the overall sample, there was a small but significant correlation between pre-post changes in both the HRSD and the BDI-II (with sleep items removed) and changes in the insomnia scores, r = .25, p < .05 and r = .21, p < .05, respectively.
At baseline 95 of the 106 participants (90%) met SCID-I criteria for insomnia, while 85 of those continued to endorse insomnia following treatment (80% of the sample and 90% of those with baseline insomnia). Similarly, 89 participants (84% of the sample) endorsed at least one of the HRSD sleep items as severe at baseline, while 62 of those continued to endorse insomnia following treatment (58% of the sample and 70% of those with severe insomnia). In terms of insomnia subtype, 51% had severe sleep initiation insomnia, 57% had severe middle of the night insomnia, and 30% had severe complaints of early morning awakening (some individuals reported more than one type of severe insomnia); the distribution of these subtypes did not change appreciably following treatment.