Sleep at sea: a diary study of the effects of unattended machinery space watch duty

Forty-nine engineer officers in the Swedish merchant marine were asked to keep a sleep/wake diary during one tour of duty (1-3 months) on ocean-going container ships. Every 2-4 nights the officers had watch duty during which they were allowed to sleep but were awakened by an automatic alarm system in the case of machinery malfunction. The diaries showed that 40% of all nights at sea were spent on such watch duty. Fifteen per cent of all nights were disturbed by alarms (and 13% for other reasons). Having watch duty, particularly with alarms, reduced ratings of sleep length (—1.5 h), sleep quality and recuperation. The latter was also negatively affected by the frequency of watch duty. Of particular interest was the fact that nights on watch were also perceived as disturbed when no alarms occurred. Such nights were also characterized by a higher level of uneasiness. It was concluded that the watch duty system significantly affected sleep and well-being, and that these effects appeared irrespective of whether any alarms occurred. The latter was interpreted as an effect of apprehension stress.