The extent to which an underground miner’s alertness is influenced by sleep deprivation, shift work, and working in elevated temperatures is currently unknown. Actigraphy and psychomotor vigilance data were collected from nineteen underground miners over 28 consecutive days. Core body temperature data was also gathered during four scheduled work shifts. Participants experienced shortened sleep durations and poor sleep efficiency throughout the study. Significant increases in reaction time occurred over the course of night shifts, a decline that was not observed during day shifts. A strong, negative correlation between core body temperature and reaction time was present throughout day shifts but did not appear during night shifts. Poor sleep and altered circadian rhythms appear to negatively affect the alertness of participants during work times. An inadequate adaptation to night work schedules may be indicated by the poor alertness and absent relationship between core body temperature and performance observed during night shifts.Master's These