Relapse rates following OCD treatment are 50%, suggesting the existence of unidentified and untreated mechanisms in OCD. One of these mechanisms relates to circadian rhythm – the body’s biological clock. One indicator of circadian rhythms is sleep timing, and previous research has found that people with OCD tend to have later sleep timing (going to bed and waking up later), which is associated with worse OCD symptoms. However, it is still unclear whether later sleep timing in OCD is due to a delayed circadian rhythm or non-circadian factors such as staying up later engaging in compulsions. This study will clarify the role of circadian rhythms in later sleep timing in OCD by measuring the associations between the timing of melatonin, desired bedtime in adults with OCD, and later sleep timing. The findings will provide important insight into whether circadian rhythms could be a new target for OCD treatment.