Stimulus control (SC) is a core component of cognitive behavioural therapy (CBT) for insomnia and is the single intervention for which there is most empirical evidence. Nonetheless, little is known about whether all of the elements within SC are critical to sleep improvement. This study, therefore, investigated the impact on sleep of the Quarter of an Hour Rule (QHR) a single, situational element considered central to SC for insomnia. The mechanisms of effect of SC intervention remain also unclear. An associated aim of the present study, therefore, was to contrast two forms of administration of the QHR to test aspects of the learning theory presumed to underlie the SC model. In addition adherence to the behavioural intervention was investigated and the possibility of using actigraphy to measure adherence objectively was explored. Prior to the randomised controlled trial (primary study), two preliminary studies were conducted. The first preliminary study aimed at determining the optimal cut-off to represent normalcy in sleep onset latency (SOL). The results indicated it to be fifteen minutes and, therefore, participants in studies two and three were asked to apply the QHR if they were not asleep within a quarter of an hour. Study two comprised three single cases and tested the feasibility of the QHR as a standalone therapy for insomnia. Visual inspection of the data and interrupted time series analyses evidenced SOL, wake after sleep onset (WASO) and sleep efficiency (S.E.) improvements in two out of three participants. Their Pittsburgh Sleep Quality Index (PSQI) score at the end of the intervention was reduced by 50% compared to baseline. The participant, whose sleep was not improved, following the intervention, had not applied the QHR. The results of this exploratory, single case, study warranted further investigation of the QHR. In study three forty-one GP and self referred volunteers, aged 18-72 years, with SOL and/or WASO complaints, formed 3 randomised groups: QHRin bed, QHRout of bed and control. Both QHR conditions required to ‘read if not asleep within a quarter of an hour’, with groups differing only with the location (in bed versus out of bed) where to apply the QHR. Sleep diary pre-treatment (two weeks) and post-treatment (three weeks), home polysomnography (PSG) (two nights pre-, two post-treatment) and sleep related questionnaire (pre and post) data were collected. Adherence with the QHR was measured objectively (actigraphy + light monitoring) and subjectively (adherence diary). Following QHR treatments, statistically significant reductions in SOL (QHRout) and WASO (QHRin and QHRout),an increase in S.E. (QHRin and QHRout) and a decrease in PSQI score (QHRin and QHRout) were found. Trends also indicated increased total sleep time (TST). Clinically significant improvements (SOL and WASO ≤ 31 minutes or reduced by 50%, PSQI ≤5 or reduced by 50%) were obtained in 33-57% of active groups participants.