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(ninc shifts across 8 d) to 12-h shifls (iwo nights-one frcc-two days-three frce) on overall alertness. How-ever, the distribution of frcc days changed at thc same limę. Two oiher studics of nurses (57) and indusirial shift workers (58) havc produced similar rcsulls. Re-ccntly Rosa & Colligan (59) used 2-h ratings in a field experimcnt and demonstrated chat the 12-h night shift, indced, produced higher ratings of fatiguc than 8-h night shifls. In addition, in a siudy of accidenis of truck drivers, Hamelin (48) demonstrated a U-shaped relation between hours driven and accidenis. ie, after an initial “warm up” period accidenl risk was Iow, with an inerease towards 11 h of driving.

As may be expectcd. also the watch-keeping Systems (4 on, 8 off) on ships arc associated with Iow alert -ness and poor performance during the night (60). Ap-parcmly, rotating systems cause greater disiurbancc lo thc individual than do stable systems.

Another lype of unusual workhours is that of air

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Knutsson et al (73) demonstrated thal the incidencc of myocardial infaretion (and cardiovascular diseasc in generał) is related to the amounl of exposure to shift work.

Finally. the trait of morningness (having a tenden-cy towards early sleep-wake prcferences) has frequently bcen associated with poor adjustment to shift work (74, 75). This has also bcen the case for the trait of sleep rigidity (76).

Concluding comments

Taken together, thc reviewed literaturę clearly ińdicates that shift work that involvc$ night shifts strongly in-fluenccs thc psychology and psychophysiology of thc individual.

crews on transmeridian routes. Then, not only are the workhours displaced to “biological” night limę, but also the timc reference is changed through time-zone shifts. As with other types of shift work. survey studics havc demonstrated disturbed sleep and wakefulness (6l). The disturbed wakefulness has been cvidcnced also in flight Simulator studics.

In somc occupations the personncl may sleep al the worksite until needed. This is the case for, among others, physicians. Since the greater part of such nights arc often are spent working, sleepiness-fatiguc is of-ten pronounced, and performance tends to be reduced, although the practical implications (for the patients) are stall unknown (62, 63). Other forms of "on-call" systems may be found among, for example, engineer officers in the merchanl marinę (64).

Among individual differcnccs age has bccn related to sleep disturbances (65, 66). In elcctroencephalo-grapliic studies trends have been found towards morę superficial sleep in middle-aged shift workers (16). The studics by Foret et al (65) and Akerstedt &. Torsvall (66) also indicatcd that expcricnce was negatively related to generał well being ovcr a number of years. Koller et al (67) found that rcduced health appeared earlier among shift workers than among day workers. Dahlgren (68) found no effeets of three years of night work on the rhythm of rated activation across night shifts. Ncither did Wynn et al (69), ovcr a temporary 10-week period of wcckly alternation between night and day work in a group of nurses. Dumont et ai (70) found that thc ainount of slcep-wakc and related dis-turbanccs in prcscnl day workers was posilivcly related to their previous cxpcricnce or night work. Guille-minault et al (71) found an overrepre$entation of former shift workers with different clinical slcep/wakc disturbances appearing at a sleep clinic. Although not directly related to skepiness, it is still of interest to ob-scrvc that Angersbach cl al (72) have demonstrated an earlier occurrence of gastrointcstinal disease among thrcc-shift workers than among day workers. Similarly,

Relerences

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