Objectives: To determine how sleep timing, quality and duration change across consecutive morning, free days and night shifts among industrial workers in an Arctic environment, and to compare these sleep characteristics between seasons with extreme photoperiods.
Methods: This observational crossover study included 111 process operators at a high-latitude (71°N) industrial plant in Norway, followed during light and dark photoperiod seasons. Of these, 81 participants participated in both seasons. Sleep was measured daily using sleep diaries and a smart ring. Participants worked a rotating schedule comprising seven consecutive morning shifts, four free days and seven consecutive night shifts. Linear mixed-effects models were used to examine the effects of shift type, season, chronotype and consecutive workdays on sleep parameters.
Results: Sleep timing and duration varied markedly by shift type. No overall seasonal effects were observed, although sleep onset and mid-sleep were delayed after the first morning shift during the dark season. Late chronotypes showed later sleep timing on free days and slept longer after night shifts. The probability of short sleep (<6 hours) was higher on both morning and night shifts than on free days. Conclusions: Both morning and night shifts were associated with shorter sleep than free days, with the greatest reduction after morning shifts. [...]