Exploring the construct of subjective sleep quality in patients with insomnia

The construct of subjective sleep quality is poorly understood. One widely used measure of subjective sleep quality is the Pittsburgh Sleep Quality Index (PSQI). The role of psychiatric illness in the association between the PSQI and a prospective, sleep diary-derived sleep quality measure (SDSQ) was investigated plus the degree to which the PSQI may reflect mood states.

A sample of 211 insomnia patients (International Classification of Sleep Disorders, Second Edition) divided by the presence or absence of a comorbid psychiatric disorder (DSM-IV-TR) and recruited between January 2004 and February 2009, completed the PSQI (primary outcome) and 2 weeks of sleep diary monitoring. First, correlations between PSQI and SDSQ were compared; second, regression analyses were used to investigate whether the association between PSQI and SDSQ depends on diagnostic status; third, the differences in sleep quality between the groups, plus the contribution of anxiety and depression in explaining these differences, were explored.

The correlation between PSQI and SDSQ was significant only in the nonpsychiatric group (P < .001). The association between PSQI and SDSQ was moderated by diagnostic status: it was weaker in psychiatric patients (P = .047). Patients with psychiatric comorbidity scored significantly higher on the PSQI than those without (P < .001); this difference disappeared after controlling for anxiety. There were no group differences for the SDSQ.

The present findings suggest that (1) psychiatric patients may be more biased in their retrospective sleep quality ratings, and (2) the PSQI total score may reflect sleep-related distress. The use of a prospective sleep diary measure in patients with a psychiatric disorder is recommended.