A study of sleep patterns on two Finnish icebreakers, ambulatory recording and automatic analysis

The effects of noise and vibration on sleep in Finnish ice-breaking ships were studied in healthy volunteer workers and a control group. EEG, EOG and EMG were recorded by means of portable tape recorders. The recordings were analysed by an automatic hybrid system. Both sleep stage parameters and the quantities of single EEG waveforms were used for the evaluation of the sleep quality. Measurements were made before, during and after the ice-breaking season in the winter. Higher amounts of wakefulness and sleep stage 1, and lower amounts of delta and theta activity were found on a night during the ice-breaking season, compared with a night after the season. This was interpreted as indicating a "lightening" of sleep, caused by noise and vibration. No differences between the nights before and during the ice-breaking season were observed. The crew members had an appr. 1 h shorter Time In Bed and Sleep Period Time than the controls also on the nights studied ashore. Because of the small number of subjects the results are not conclusive, even though statistically significant differences were obtained. The delta activity in seconds/minute seems to be a more informative measure than the percentages of the sleep stages S3 and S4. The possibility of using the theta activity as a measure of the length of sleep should be further investigated.