Observing Sleep And Cardiovascular Responses During Mountain Warfare Training

During the Mountain Warfare Training Exercise (MTX), a multi-week field training performed at elevation (range: 2100-3500 m), ~10% of Marines drop from the course due to physical deterioration. Such deterioration may be related to the effects of the environmental conditions and training pace on sleep and recovery. Disturbed sleep and cardiovascular stress (a marker of recovery) may be exacerbated during the extreme cold of the winter months and further hinder the ability to recover in these austere environments. PURPOSE: Evaluate differences in sleep and cardiovascular stress between temperate and cold seasons during MTX. METHODS: Seventy-two (n = 72) Marines (mean ± SD, age: 22 ± 4 yr, height: 174 ± 7 cm, weight: 79 ± 11 kg) participated in temperate (n = 26) and cold (n = 46) MTX training events. Mean ± SD air temperature for each MTX was 11 ± 2 °C (temperate) and -5 ± 4 °C (cold). Periodic check-ins occurred at the Start, Midpoint, and End of each MTX, with a final check-in (Post MTX) performed after Marines had returned to their home base (sea level). Heart rate (HR), heart rate variability (HRV), total sleep time (TST), and sleep efficiency (SE) were recorded via a commercially available finger-worn biometric monitor. One-way ANOVAs were used to compare metrics across MTX check-ins and seasons (p < .05). RESULTS: Significant check-in by seasonal effects were found for TST and SE (p < .01 for both). In the temperate MTX, TST was shorter at Start (5.8 h) and End MTX (6.0 h) than at Midpoint (6.6 h) and Post MTX (6.4 h), while SE was lowest at End MTX (78%) than at the other check-ins (all >80%). TST and SE did not change significantly during the cold MTX. For cardiovascular responses, a significant effect of check-in was found for HR during sleep (p < .01), as HR increased by 4 bpm at Midpoint (62 bpm) and returned to baseline levels by Post MTX (58 bpm). No other significant check-in, seasonal, or interaction effects were found for HR or HRV. CONCLUSION: During the temperate MTX, Marines experienced worse sleep (i.e., shorter and more disrupted) during the second half of training, but this was not seen during the cold MTX. This seasonal variation was not observed in cardiovascular responses, suggesting poor sleep did not impact cardiovascular stress. Further work is needed to examine the functional implications of these outcomes.