Insomnia and depression: How much is the overlap?

Previous literature considers insomnia as one of the features, predictor, and also as a residual symptom of depression. However, chronic insomnia and major depressive disorder (MDD) have overlapping features making differentiation between two difficult.

Forty subjects in each of the three categories-MDD, insomnia (I) and combined diagnoses (MDD-I) were recruited in this study after excluding potential confounders. Diagnosis of MDD was made following Diagnostic and Statistical Manual 5 edition (DSM-5), while the International Classification of sleep disorders 3 edition criteria of insomnia were used for diagnosing insomnia. The severity of insomnia and depression was assessed using the Insomnia Severity Index (ISI) and Patient Health Questionnaire-9 (PHQ-9), respectively. Fatigue was assessed using the Fatigue Severity Scale (FSS), which was translated in Hindi for this study. All subjects were also asked regarding effect of good sleep at night on daytime symptoms, especially on mood.

Subjects in MDD group were younger than the other two. Insomnia group was significantly different from the other two groups on most of the measures according to the DSM-5 criteria for MDD. MDD group had lesser frequencies of initial insomnia, middle insomnia, dissatisfaction with sleep and overall distress during the day. MDD-I group had a higher prevalence of daytime sleepiness and hyperactivity/impulsivity. PHQ-9 score was the lowest in the insomnia group. Despite statistically significantly different, ISI score was clinically comparable. The severity of fatigue was comparable across three groups. Contrary to the MDD group, subjects in insomnia and MDD-I group reported significant improvement in daytime symptoms after a good sleep for even one night.

There is considerable overlap of symptoms between insomnia and MDD. Subjects having insomnia report significant improvement in daytime and mood symptoms after good sleep, contrary to subjects with MDD.