One means of assessing the hazard risk associated with mine work is to study the severity of injuries that occur during the workday. Of special interest is the accident risk inherent in night work and rotating shiftwork. To better understand this risk, the U.S. Bureau of Mines studied accidents that occurred during a 10-year period in the taconite (iron ore) mining operations of the U.S. Lake Superior iron ore region. In general, accidents that occurred during the night shift resulted in more days lost per accident, compared with either the day or afternoon shifts. To control for the possibility of different accident types occurring on different shifts, part of this study focused on accidents involving equipment operation. Again, night shift accidents were shown to be significantly more severe than on the other two shifts. These results implicate work performance during night hours as being relatively impaired, perhaps owing to lowered states of psychophysiological arousal, coupled with the handicap of operating in a darkened environment.