Sleep and fatigue-related performance impairments are widely recognized occupational hazards, which are common in the offshore oil and gas industry. Our study among rotating (2-weeks-on/2-weeks-off) offshore day-shift workers found that the prevalence of fatigue among offshore workers was high (73%), with 1 in 4 workers experiencing severe sleepiness each day on shift. Offshore, workers accumulated >2.5 nights of consolidated sleep loss and fatigue increased with days-on-shift. Furthermore, we identified three potential fatigue risk prone periods in which fatigue accumulated. An end-of-shift effect, as fatigue was highest in the post-shift measure and increased with days-on-shift; a third quarter phenomenon, as fatigue scores peaked thre...