Effect Of Sleep And Shift Characteristics On Heart Rate Variability In Wildland Firefighters

Abstract

Wildland firefighting requires demanding physical work while exposed to harsh working conditions, including long working hours, sub-optimal sleep, and psychological stress. These stressors may induce autonomic nervous system imbalance, which indicates potential risk to worker health and safety. PURPOSE: This pilot investigation examined the effect of sleep and shift characteristics on variables related to heart rate variability (HRV). METHODS: A within-subject, observational study was conducted on 4 wildland firefighters (3M, 1F) between July and August of the 2021 fire season. HRV was measured before and after shift using a chest-worn monitor (Polar H10 + Ignite Activity Tracker). Sleep variables were measured subjectively via sleep log and objectively via wrist-worn actigraphy. Subjective sleepiness and fatigue were also measured via 7-point scales (Stanford Sleepiness Scale; Samn-Perelli Fatigue Scale). Pearson correlation and linear regression analyses were performed to test if subjective, shift or sleep variables predicted HRV variables. RESULTS: Mean RR interval was positively related (r=.37) and significantly predicted by objective sleep quality (R2=.14, F(1, 59) = 9.24, p < .005), while also negatively related (r=-.19) and significantly predicted by subjective sleepiness (R2 = .03, F(1, 113) = 4.07, p < .05). rMSSD was negatively related (r=.-.23) and significantly predicted by wake-up time (R2 = .05, F(1, 75) = 4.06, p < .05). Surprisingly, total HF power was negatively related (r = .-26) and significantly predicted by start time (R2=.07, F(1, 66) = 4.92, p <.05), meanwhile LF/HF ratio was positively related (r=.27) and significantly predicted by start time (R2= .07, F(1, 66) = 5.06, p<.05). CONCLUSION: HRV measures were related to and significantly predicted by certain subjective, sleep, and schedule variables, thus warranting further investigation.