Our earlier studies on board naval ships with two-watch systems have indicated relatively high levels of sleepiness, especially during nighttime. The present study was made on board a naval surface vessel using a three-watch system (4 hours on, 8 hours off, with three teams) during a longer journey. Subjective sleepiness was measured using the Karolinska Sleepiness Scale and subjective sleep parameters using a sleep diary. Both methods have been validated. Ratings were obtained at change-over times. The results were means of watches/off-duty watches disregarding temporal order. Results were obtained from 21 persons. Sleepiness was fairly high in connection with watches during the period between 00h and 08h. The three watch teams did not differ in overall levels of sleepiness. Sleep duration was adequate for all three-watch teams, i. e., around 8 hours. Sleep quality was good for all participants. The most frequent sources of sleep disturbance were noise and high sea. In summary, the nighttime trough in alertness depending on the diurnal rhythm was not completely avoided even with a three-watch system. The latter system is probably to prefer since shorter watches decrease the risks for sleepiness related mistakes. In addition, the system seems to be equally "fair" or "unfair" to all watch teams.