When performing night work the sleep-activity cycle is delayed by 8 to 9
hours. The physiological rhythms follow this delay with various speeds and
damped amplitudes. Complete adaptation (resynchronisation of physiological
rhythms with the deliberately shifted sleep-activity cycle), which is observed
in some persons, takes approximately 1 day per shifted hour. But many,
especially morning oriented persons are unable to adapt and their physiological
rhythms remain dissociated throughout the entire night work period. This
dissociation which occurs with each change from day to night work is assumed to
contribute to the genesis of multifactorial diseases, in particular to
cardiovascular ones.
A focal concern of this research area is on one hand the acceleration of the
adaptation to night work by moderately elevated light intensity at the
workplace, on the other hand whether and under which conditions this measure
could be effective in persons that are known to be intolerant against night
work. Another central question is whether this intolerance is endogenous or
related to an unfavourable interaction between shift hours, individual phase
position and light at the work place.
Indicators of adaptation are the temporal parameters of melatonin and of
cortisol, which are both secreted in a strong circadian rhythm. First results
show that the application of light induces a significant adaptation to night
work even in persons that are intolerant against night work. The degree of
adaptation is still smaller in morning- than in evening- and in neither types
and associated with a shortening of the cortisol quiescent period accompanied by
an overall elevated cortisol production as well as a stronger dissociation of
the physiological rhythms. These observations lead to the hypothesis that
morning types are under a higher risk to develop cardiovascular diseases when
frequently performing night work in the long run. This will now be substantiated
in further studies.