Night Work During Midnight Sun and Polar Nights: Impact on Alertness, Sleepiness and Fatigue

Introduction: The circadian clock's sensitivity to light means that seasonal changes in photoperiod can affect circadian entrainment in shift workers, thus influencing their adaptation to night work. At high latitudes, such as the Arctic, extreme daylight variations across seasons, provide a unique context for studying such circadian disruption. Polar nights and midnight sun may further challenge circadian entrainment mechanisms. Whether shift work and seasonal variations cumulate or combine their effect in the photoperiodic zeitgeber remains unclear. This study investigates how seasonal variations may interact with consecutive morning and night shifts to impact alertness, sleepiness and fatigue.

Methods: We recruited 112 three-shift process operators at 71°N, among which 84 participated during both the dark and light seasons. Their work schedule included blocks of seven consecutive morning shifts and seven consecutive night shifts, separated by four rest days. Alertness was measured by the 3-minute Psychomotor Vigilance Task (PVT-B), using lapses (omissions ≥355ms) and response speed (inverse response time) as outcome variables. Sleepiness was assessed by the Karolinska Sleepiness Scale (KSS). A separate question was used to assess mental fatigue. Tests were conducted at the end of the operators’ shift on days 1, 3, and 6 of each shift block. KSS was also assessed at the start and end of all shifts. Data were analyzed using multilevel mixed-effects regression models with season, shift type (morning/night) and consecutive workday number as fixed effects.

Results: Night shifts were linked to lower alertness, as well as higher sleepiness and fatigue, especially on the first night shift. Interaction effects indicated improved response speed, fewer lapses, and reduced sleepiness and fatigue over six-night shifts, with significant differences between night 1 and night 6. These patterns were similar for both seasons across all variables. However, PVT-tests showed that the overall contrast between morning and night shifts was only significant during the dark season (response speed: p < 0.0001, lapses: p≈0.005), but not during the light season. Neither sleepiness, nor fatigue, were significantly affected by seasons.

Conclusion: These findings suggest that while night shifts generally impair alertness, as well as increase sleepiness and fatigue, workers may gradually adapt over consecutive shifts, as evidenced by improvements by the sixth night. The lack of significant seasonal effects on sleepiness and fatigue suggests that workers might adapt equally well to night shifts regardless of the photoperiod. However, the more pronounced differences in alertness between morning and night shifts during the dark season highlight the potential interplay between reduced daylight’s influence on circadian rhythms and adaptability to work night shifts.

 

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Artikel online veröffentlicht:
08. Oktober 2025