Effect of accumulated night work during the working lifetime,
on subjective health and sleep in monozygotic twins
MICHAEL INGRE 1 , 2 and TORB J ÖRN ÅKERSTEDT 1 , 3
1National Institute for Psychosocial Medicine (IPM); 2Department of Psychology, Stockholm University and 3Karolinska Institutet, Stockholm,
Sweden
Accepted in revised form 17 December 2003; received 14 November 2003
SUMMARY The present study sought to investigate the effects of lifetime accumulated night work
on subjective health and sleep in monozygotic twins discordant on the exposure to night
work. Atotal of 169 twin pairs (83 males, 86 females) over 65 years of age were
analyzed on variables that describe the presence or absence of complaints regarding
subjective health, disturbed sleep, repeated awakenings and not being well rested.
Significant (P < 0.05) risk ratios (RR) were found for subjective health (RR ¼ 1.67)
and disturbed sleep (RR ¼ 2.83) indicating a higher risk of developing complaints to
former night workers. There was neither a dose-dependent effect of night work nor an
interaction effect with gender observed.
key w ords
health, lifetime accumulated night work, sleep, twins
INTRODUCTION
One of the major health problems in shift work is disturbed
sleep (Åkerstedt 2003). Anumber of studies have shown that
particularly the night shift is associated with reports of sleep
problems (Aanonsen 1964; Graf et al. 1958; Pilcher and
Coplen 2000; Ribet and Derriennic 1999; Thiis-Evensen
1958). Polysomnographical studies have shown that sleep is
reduced by about 2 h after a night shift or before a morning
shift but is of normal length after an evening shift (Åkerstedt
et al. 1991; Foret and Benoit 1974; Tilley et al. 1982). Most of
the loss seems to affect stage 2 and REM sleep. The problems
of sleep (and wakefulness) in shift work have led authors to
suggest the existence of a shift work maladaptation syndrome
or a shift work sleep disorder (Guilleminault et al. 1982; Lavie
et al. 1989).
It also seems that sleep problems may increase with
acc