Multi-day single-handed sailing races put exceptional strain on sailors, requiring high cognitive functioning and 24 h per day readiness to perform. Fatigue from sleep loss, circadian misalignment, workload and other factors is of significant concern, jeopardizing competitiveness as well as safety. Almost no research has been devoted to this, in part because collecting data on sleep and performance in solo sailors during races at sea is challenging. The present study aimed to contribute valuable data on the issue by assessing sleep–wake patterns and functional impairments in a total of 16 sailors during a two-leg transatlantic race. Each sailor recorded sleep periods and functional impairments during the two legs of the race. Self-reported sleep duration per 24 h was 4.1 ± 0.4 h in the first (shorter) leg and 4.6 ± 0.4 h in the second (longer) leg. Sleep was polyphasic and varied in a normal circadian pattern – sleep propensity was highest (about 50%) in the middle of the night, with a smaller (nearly 15%) secondary peak in the middle of the afternoon. Significant functional impairments were reported throughout the race including technical errors, mood changes and hallucinations. These impairments are consistent with the typical effects of substantial sleep loss and are likely to reduce the safety margin. Single-handed sailors could benefit from the development of innovative tools to help them to manage sleep and fatigue and thereby improve safety and effectiveness.