© 2014 Elsevier Ltd and The Ergonomics Society. In addition to long contractual hours during offshore weeks (14×12h shifts), many personnel on North Sea oil/gas installations also work overtime, but little is known about the implications of overtime for sleep patterns offshore. In this study, the additive and interactive effects of overtime and age were analysed as predictors of sleep duration and sleep quality among offshore day-workers (N=551), 54% of whom reported overtime. Sleep duration and quality were impaired among personnel who worked overtime, relative to those who worked only standard shifts; there was also an inverse dose-response relationship between overtime hours and sleep duration. Although the sleep measures were more favourable during shore leave than during offshore weeks, there was little evidence of compensatory sleep patterns. These findings are discussed with reference to known performance and health effects of short sleep hours; formal guidance on overtime work offshore is noted; and methodological issues are considered.