During the year 2006 work with the computerized simulation software, SWP (Sleep-Wake predictor) has been ongoing. A new detailed instruction has been written. A new function enabeling time zone crossings ("jet lag") to be entered as a parameter into model simulations. Field studies on the effects of sea watch systems have been carried out. This is summarized below. o Sleepiness is always experienced during the night. This is mainly due to the diurnal rhythm. o The possibility to get sleep is also dependet on the diurnal rhythm. Sleep during the night is always "easier" than during the day. o The diurnal rhythm is stable and cannot be changed in a short time! o Loss of sleep always leads to fatigue. 20-24 hours without sleep leads to degratation of performance compareble to that experienced in connection with drunk driving. o In summary, working at night after to short preceding sleep will lead to serious, negative consequenses for performance and safety. o Sea watch systems disregarding the type will lead to increased levels of nighttime sleepiness. o Noise and rough seas will disturb sleep. Recommendations o The 4 on/4 off system may be recommendated if the work on board is physically demanding (including rough seas - specially affecting smaller vessels). o "dog watches " may be used to achieve similar "stress" on both watch teams in a two-watch system. o An alternative is to postpone watch change over times with 3 hours (from, e.g., 00, 06, 12, 18 hours to 03, 09, 15, 21 hours). Thereby, sleep during off-duty watches is facilitated at the same time as the period of extreme night time sleepiness is curtailed (for those on duty). o The above recommendations apply mostely to ahorter missions (approximately up to 7 days). For missions longer than that a three-watch system is recommended (i.e., three watch teams alternating on a 4 hour on/8 hour off system). A nonalternating 6on/6off system may be suitable on submarines (since there is no space for a larger crew. It should be noted, once again, that noise and rough seas will desturb the possibility to get sufficient rest in connection with all watch systems. o If possible, all off-duty watches should be used for rest/sleep - i.e., not to be disturbed by other duties. o Ships differ in size and mission. The commanding officer has the responsibility to, within the framework of the mission, to adjust the watch system and the possibilities to rest/sleep to the biological needs.