study question: Are daily cycles in urinary melatonin and oxidative stress marker levels (8-hydroxydeoxyguanosine) altered in PCOS, and is this associated with changes in sleep quality? summary answer: There is an association between elevated nighttime melatonin and 8-hydroxy-2-deoxyguanosine (8-OHdG) levels, and poor sleep quality in our PCOS study group. what is known already: Women with PCOS are known to have poorer sleep. However, there have been few studies examining the possible association between melatonin levels and sleep quality in women with polycystic ovarian syndrome (PCOS). study design, size, duration: This is a case-control study of PCOS (n = 26) and non-PCOS control (n = 26) subjects recruited from a tertiary gynaecological centre. participants/ materials, setting, methods: The participants were requested to complete sleep questionnaires for a month. In a subgroup from these cohorts (PCOS, n = 15; controls, n = 18), urine samples were also collected at various time points over a 24-h period. In addition, their sleep patterns and lighting environment were monitored for 3 consecutive days and nights using a wrist-mounted Actiwatch device. main results and the role of chance: PCOS women had significantly elevated night-time urinary levels of the melatonin metabolite 6-sulfatoxymelatonin (aMT6s) and of 8-OHdG (both at P , 0.05), as well as significantly reduced sleep quality (P , 0.05), compared with the controls. limitations, reasons for caution: Due to the small sample size of the study, further studies will be required to confirm our findings. wider implications of the findings: Our preliminary work provides a possible new insight into the interactions between melatonin, increased oxidative stress and sleep in women with PCOS. study funding/competing interest(s): The study was funded by the Faculty of Medicine, University of Southampton.