This study examined the shift parameters that contribute to sleep loss and on-duty fatigue in British Columbia Wildfire Service (BCWS) firefighters using sleep-wake data, work-rest data and alertness and fatigue predictions from a biomathematical model (BMM) of fatigue. A total of 40 firefighters (age: 30.4 ± 11.6 years; 13 F, 26 M) volunteered over a 14-day consecutive fireline deployment, followed by a 3-day rest period, at two separate fires in British Columbia (during the 2015 fire season). Sleep–wake data were obtained using a wrist-worn accelerometer and self-reported sleep logs. Shift start and end times were provided by the BCWS at the completion of the study. Sleep and shift data were manually entered into a validated BMM (Circadian Alertness Simulator) to generate fatigue scores and shift work patterns. Shift duration was the major contributor to fatigue, as 46% (n = 274) of shifts were ≥14 h in length and the average shift length was 13.0 ± 0.62 h. However, none of the firefighters had a high-risk fatigue score (>60). The findings from this study indicated that using a BMM of fatigue can provide important insights into shift-work parameters that contribute to workplace fatigue and sleep loss in wildland firefighters.