Effect of Placebo Conditions on Polysomnographic Parameters in Primary Insomnia: A Meta-Analysis

Little is known about the role of placebo response in the pharmacotherapy of primary insomnia, especially about the effect of placebo intake on objectively assessed outcome variables. Our aim was therefore to conduct an effect-size analysis of placebo conditions in randomized controlled drug trials addressing primary insomnia also including polysomnography.

We conducted a comprehensive literature search using PubMed, PsycINFO, PSYNDEX, PQDT OPEN, OpenGREY, ISI Web of Knowledge, Cochrane Clinical Trials, and the World Health Organization International Clinical Trials Registry Platform. The meta-analysis used a random effects model and was based on 32 studies reporting 82 treatment conditions covering a total of 3,969 participants. Special emphasis was given to the comparison of objective and subjective outcomes and the proportion of the placebo response to the drug response.

Effect sizes estimates (Hedges g) suggest that there is a small to moderate yet significant and robust placebo response reducing the symptoms of insomnia in terms of sleep onset latency (-0.35), total sleep time (0.42), wake after sleep onset (-0.29), sleep efficiency (0.31), subjective sleep onset latency (-0.29), subjective total sleep time (0.43), subjective wake after sleep onset (-0.32), subjective sleep efficiency (0.25) and sleep quality (0.31). Thus, the placebo response was also evident in objective, physiological (polysomnographic) variables. Our results indicate that 63.56% of the drug responses are achieved even in the placebo groups.

In light of these strong placebo responses, future studies should investigate how to exploit placebo mechanisms in clinical practice.