Circadian adjustment of men and women to night work.

OBJECTIVES: The aims of the present study were to define the adjustment of sleep-wakefulness and other circadian rhythms during consecutive night shifts and to study the effect of gender on the adjustment to night work.

METHODS: Twenty experienced shift workers, 9 men and 11 women, were studied under controlled laboratory conditions. After a night of habituation, the subjects worked one day shift and three night shifts. Rectal temperature was measured continuously during the study. Salivary melatonin and cortisol were assessed at 2-h intervals, and subjective sleepiness was checked every hour during the work shifts. Sleep was registered by the static-charge sensitive bed (SCSB) method after the workshifts.

RESULTS: Subjective sleepiness varied both between the work shifts and between the men and women. The subjects felt most alert during the day shift and were the sleepiest during the first night shift. Sleepiness decreased during the consecutive night shifts but did not reach the level of the day shift during the three nights. The sleepiness of the women decreased more than that of the men, the difference being significant during the second and third night shifts (P < 0.05). The circadian rhythm of body temperature, salivary cortisol, and time in bed changed significantly (P < 0.05) between the workshifts, but no differences were found between the men and women in the circadian adjustment of the physiological variables to night work.

CONCLUSIONS: The men felt sleepier than the women during the consecutive night shifts, although no physiological differences were found between the genders. However, since normal social and domestic factors were excluded, the findings cannot be considered as evidence of women's better adaptation to night work in everyday life.