The relationship between sleep, fatigue and performance during sub-Arctic operations: data from a winter military...

Military operations in cold environments impose significant physiological and psychological stress, resulting in fatigue and impaired cognitive and physical performance. Sleep, a critical factor for recovery, is often disrupted in field conditions, yet its relationship with fatigue and task performance under different operational roles and living conditions remains understudied.

This study investigated perceived fatigue (10 cm Visual Analogue Scale), physical performance (standing long jump), sustained attention (20-minute psychomotor vigilance task (PVT)) and sleep (actigraphy) in first-year and second-year cadets during a winter military exercise in Finland. Cadets were assigned roles reflecting combat (first-year) or officer duties (supervision, coordination; second-year) and housed in either heated tents or barracks. Training workload and environmental conditions including temperature and air quality were recorded across two training blocks differing in intensity.

First-year cadets accumulated significantly more physical activity (p=0.005) and obtained less total sleep per night compared with second-year cadets, with more second-year cadets achieving >5 hours of sleep (p=0.017). No significant difference was found in the number of cadets obtaining >7 hours of continuous sleep. Perceived fatigue increased post-first training block only in first-year cadets (p=0.003). First-year cadets demonstrated a notable decline in PVT reaction time (p<0.001) and jump performance (p=0.001), with no recovery after the second block. Second-year cadets also showed a PVT decline (p<0.001) but exhibited partial recovery (p=0.001). Increased fatigue correlated negatively with PVT performance (all p<0.043). Accommodation in tents showed large temperature fluctuations (~30 °C) and elevated particulate matter (PM10); barracks experiencing increased CO₂ proportional to occupancy.

A challenging sleep environment combined with high training loads impairs fatigue resistance and performance during cold-environment military training. Sleep continuity is crucial for fatigue and in turn sustained attention. Long (~20 min) sustained tasks might serve as sensitive fatigue monitoring tools. Improvements to sleep environment such as increased temperature comfort might improve sleep, reducing fatigue.