Effect of age on sleep onset time in rotating shift workers.
BACKGROUND
To determine the relationship between sleep onset time and age in rotating shift workers.


METHODS
Sleep diaries were used to record the sleeping onset time in rotating shift workers. Work shifts were rotated on a weekly basis and fell into three periods: morning (06:00-14:00 h), evening (14:00-22:00 h), and night (22:00-06:00 h). Work shifts were rotated in the following order: night, evening, and morning. One working week consisted of 5 days. The mean age of the male shift workers was 40.3 years.


RESULTS
A significantly earlier sleep onset time was observed in older workers working morning and evening shifts (r=-0.42 and r=-0.66, respectively), but not when working night shifts (r=-0.10). Regardless of age, night-shift workers usually go to sleep after their shift ends at 06:00 h. After the evening shift ends at 22:00 h, older workers tend to go to sleep earlier than younger workers.


CONCLUSIONS
Sleep onset time becomes earlier with age in morning- and evening-shift workers. The morning shift starts very early, at 06:00 h, so workers must go to sleep very early to obtain an adequate amount of sleep. Older workers may go to sleep earlier because of physiological (circadian) and/or social factors associated with shift work.
Department of Public Health, Gunma University School of Medicine, Showa 3, Maebashi 371-8511, Japan. kawada@med.gunma.ac.jp
Tomoyuki
T
Kawada