Introduction
Mental contrasting with implementation intentions (MCII) has been successfully applied to improve health-related behaviors (e.g. exercise). We explored its effectiveness to improve sleep outcomes beyond effects of sleep hygiene (SH) information, and investigated associations with stress. Eighty university employees (mean age: 29.6, SD = 4.5) were randomized to either a MCII + SH or a SH-only condition. During a baseline-week and a post-intervention week, sleep duration (Fitbit Alta and self-report), sleep quality, and stress were assessed daily and saliva was collected to assess the cortisol awakening response (CAR). In total, self-reported sleep quality and duration increased, but there was no meaningful condition*week interaction for sleep parameters or CAR. Higher average stress was associated with shorter sleep duration and lower sleep quality. Within-person, days with higher stress were followed by nights with lower sleep quality. Despite overall improvements, effects of MCII were not confirmed. MCII might be less effective to improve behaviors which are less controllable.Insufficient sleep is a widespread problem and the consequences are well documented and far-reaching. For adults between 18 and 64 years, the National Sleep Foundation (NSF) generally recommends a range of nightly sleep duration from 7 to 9 hours (Psychological interventions to improve sleep in academia very rarely assess objective sleep outcomes and largely focus on early undergraduate or college students (Existing intervention studies aiming to improve sleep in healthy adults most commonly provide information on the importance of sleep or on behavior promoting good sleep, so-called sleep hygiene practices (