Open-label placebo for insomnia (OPIN)

Participants with insomnia symptoms are initially recruited to a large observational cohort to measure changes in insomnia symptoms over a 4-week period as an outcome variable. After two weeks of this observation period, participants will be randomised to one of three different arms. Participants randomized into arm 1 will receive an invite to trial open-label placebo, those in arm 2 will receive an invite to trial conventional placebo, and a third arm will receive no invite (observational control). In both arm 1 and arm 2, the investigational product is placebo capsules (i.e. capsules containing microcrystalline cellulose). Participants are instructed to take 2 capsules (oral) per day for 2 weeks, 10-15 minutes before they go to bed. Participants in arm 1 (open-label placebo) are informed that the capsules are placebo capsules (i.e. they contain an inert agent), but provided with a brief rationale as to how placebos may be effective. Participants in arm 2 (conventional placebo) are informed that the capsules are a new pharmacological agent [7 digit code name] that can reduce insomnia symptoms, and provided with a brief rationale as to how this drug may be effective. Adherence with placebo capsules will be assessed by asking participants to self-report whether and when they took their capsules each day, as part of a sleep diary. Participants will also be asked to return any unused capsules to their final study site visit.