Mechanisms of change in an online acceptance and commitment therapy intervention for insomnia.

Insomnia, i.e., difficulty falling asleep or staying asleep, is a common condition that is connected to many psychological and physical problems. Online-based Acceptance and Commitment Therapy (ACT) has recently been introduced as an option for treating insomnia. However, our understanding is yet limited on what happens during an online ACT intervention or what underlying mechanisms are critical for outcomes. This study addressed this gap by investigating mediators of a brief self-guided online ACT intervention for adults suffering from subclinical and clinical insomnia. A total of 86 adults were randomized to an intervention group (n = 43) or a waitlist control group (n = 43). Mediator models were used to investigate the effects of online ACT on subjective sleep complaints through changes in daytime sleepiness, dysfunctional beliefs, psychological symptoms, mindfulness, and thought suppression. Two models showed significant indirect effects: The online ACT intervention decreased participants' thought suppression and depressive symptoms, which then decreased subjective sleep complaints. Other models did not yield significant mediating effects. Acceptance and mindfulness-based approaches may serve as viable options for other existing insomnia treatments. Future studies are encouraged to be conducted, especially concerning flexibility and inflexibility processes as possible mechanisms of change in online ACT for insomnia.