Ships typically operate on a 24/7 basis giving rise to a wide variety of working time arrangements and watch keeping schedules. Typically, these can be divided into 2-watch systems (where two watch keepers/teams share the 24-h period) and 3-watch systems (where three watch keepers/teams share the 24-h period). The current study uses the three-process model of alertness regulation to compare these systems in terms of the amount of severe sleepiness that is predicted to take place on watch and the amount of sleep that is predicted to occur while off watch. Separate predictions are calculated for individuals characterised as morning and evening chronotypes. Comparing 2-watch systems, highest levels of severe sleepiness were seen for evening types working 0000-1200 within the 12on12off system. The longest sleep per 24 h day was also found for evening types, but for those working the 1200-0000 watch within the 12on12off system. Total daily sleep duration ranged between 268 and 445 min. However, the picture is complex and the lowest risk of severe sleepiness while on watch is not necessarily correlated with the maximal time available for rest and recuperation when off watch. For 3-watch systems, the five-and-dime system (changeover times: 02-07-12-17-22) stands out having the lowest prevalence of severe sleepiness on watch and the longest amount of predicted daily sleep off watch. Considerable differences exist between morning and evening types offering the opportunity for considerable improvement in sleep amount for fixed (but not rotating) systems when individual chronotype is considered in watch scheduling. It is concluded that 3-watch systems, although economically costlier, have clear advantages over 2-watch systems, but that a perfect system that fits all does not exist due to the considerable impact of individual differences related to sleep/wake regulation.