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Long working hours and sleep disturbances: the Whitehall II prospective cohort study.

STUDY OBJECTIVE
To examine whether exposure to long working hours predicts various forms of sleep disturbance; short sleep, difficulty falling asleep, frequent waking, early waking and waking without feeling refreshed.


DESIGN
Prospective study with 2 measurements of working hours (phase 3, 1991-1994 and phase 5, 1997-1999) and 2 measurements of subjective sleep disturbances (phase 5 and phase 7, 2002-2004).


SETTING
The Whitehall II study of British civil servants.


PARTICIPANTS
Full time workers free of sleep disturbances at phase 5 and employed at phases 5 and 7 (n = 937-1594) or at phases 3, 5, and 7 (n = 886-1510).


MEASUREMENTS AND RESULTS
Working more than 55 hours a week, compared with working 35-40 hours a week, was related to incident sleep disturbances; demographics-adjusted odds ratio (95% CI) 1.98 (1.05, 3.76) for shortened sleeping hours, 3.68 (1.58, 8.58) for difficulty falling asleep; and 1.98 (1.04, 3.77) for waking without feeling refreshed. Repeat exposure to long working hours was associated with odds ratio 3.24 (1.45, 7.27) for shortened sleep, 6.66 (2.64, 16.83) for difficulty falling asleep, and 2.23 (1.16, 4.31) for early morning awakenings. Some associations were attenuated after adjustment for other risk factors. To a great extent, similar results were obtained using working hours as a continuous variable. Imputation of missing values supported the findings on shortened sleep and difficulty in falling asleep.


CONCLUSION
Working long hours appears to be a risk factor for the development of shortened sleeping hours and difficulty falling asleep.