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Figuro 1. Mean orał lempcrajure ot >50 shilt workers during the lirsl and fłflh night shifts The 24-h period with the first night Shift began as a norma! day with awakcning around 0700-0800 atier a nighfs sleep {lilled bar) bul contains an af-' irnoon nap around 1500 (lilled bar). (Figurę redrawn trom refcr-ence 4|

time of day

rect changcs in the rest-aciivity pattern. This holds true for, amongothers, noradrenalinę excretion, heart ratę, and blood pressure. Incidcntally, a rather peculiar ob-servation in this context is that the subjective effort associated with a certain hean ratę at a given work load is higher during the night shift than during the day shift (5, 6). In some sense this phenomenon could be inter-preted as the subjects bcing '‘oldcr’’ on the night shift. Maximum work capacity does not differ (hough. A somewhat related observation is the occurrence of vemrieular eciopic activity in conncction with night work (7).

With rcspect to adjustment over several consccutivc shifis figurę 1 suggests that a few hours* delay of the nightly fali of orał temperaturę has occurred by the fifth night shift. Still, the minimum occurs al the same limę as during the first night shift. If at all present, the adjustment over the five night shifts musi bc con-sidered marginal. The same paltem has bccn ob$erved in many other studies (3). It is likcly, howevcr, that part of the apparent adjustment is a direct effcct of the cnvironmcnt, unrelated to the biologicai clock but ‘‘masking’’ its output (8). Lying down will, eg, reduce body temperaturę, and activity will raise it, both mask-ingthe underlyingcircadian pattern. Aciually, it might be argucd that the endogenous circadian rhythm ncver adjusts in shift workers (8). The reason for the marginal or nonexisting adjustment is that the circadian system, as discussed later, is very persistent and needs a longer time for adjustment than night workcrs evcr cnjoy sińce they usually revert to a diurnal life when off duły.

Laboratory studies allow a much bclter control of environmcmal infiucnces and make it casicr to carry out around-the-clock measurements. In one of thcclas-sic studies Colquhoun et al (9) showed that orał temperaturę across 12 consecutive night shifts flatlened but never complctcly adjusted. Similar resulls ha\*c bccn published by. cg. Knaulh ct al (10) and Wcitz man & Kripkc (II). On the whole, most of (he adjust-mcnt tends to occur during the first I to 3 d and then proceeds at a slower pace. I( should be observed that in these sludies all cnvironmcntal synchronizers (light, food, social life) weregcared towards a noclurnal life. This is something the night worker has little chance to expericnce.

It should be emphasized that most of the studies of the physiological circadian rhychms of shift workcrs are mainly of theorctical intcrest sińce a elear relation between rhythm adjustment and healih parameters has seldom becn demonstrated, except for a few studies suggesting that individuals who have difflcultics tolcr-ating shift work may havc desynchronized rhythmsor smali amplitudes of their entrained rhylhms (12).

Sleep

Disturbcd sleep is perhaps the most dramatic effect of shift work. A number of survcy studies have indicated that shift workers havc difftcullics mainly at main-taining sleep after the night shift and initiating sleep before the moming shift (13). The afternoon shift has usually presented no sleep problems.

The standard psychophysiological approach to sleep usually involvcs rccording an eleclroencephalogram, an electrooculogram, and an electromyogram on pa-per and scoringthc output visually in sleep stages per 30-s intervals (14). The standard sleep stages include wakefulness (stageO), superfkial to decp sleep (stages I to 4), and rapid eyc movemcni sleep (stage REM — dream sleep).

Sleep studies of shift workers havc mostly bcen car-ried out in the laboratory (13). Rcccmly, however. some studies of shift workers’ sleep havc bccn madę in the workcrs’ natural sleeping environment (15—17). The results are fairly conclusive in that sleep length on the night and morning shifts of rotating shift workers is reduccd by I to 4 h. This rcduction mainly af-fects stage 2 and REM. Stages 3 and 4 [which together make up slow wavc sleep (SWS) or decp slccpl seem seldom to be affccted. Furthermore, sleep latency is inereased in conncction with the morning shift and is shortened in connection with the night shift. Figurę 2b dcmonstrates a hypnogram (sleep stages plottcd against time) for the night shift. Notę that the post-workday sleep is short but otherwisc exhibits a nor-mal pattern with Iwo sleep cycles.

Rather little is known aboul (he adjustment process across a scries of night shifts. The availabk studies sug-gest (hal sleep length does not improve a great deal (18, 19). Pcrmanent night workcrs seem to sleep longer. howcvcr, (han rotating shift workers on the night shift (19-23).

The reason for the shortened daytimc sleep has in several studies been attributed to higher noise levcts at that time (24, 25). This may certainly bc one of the causcs of disturbcd daytimc sleep. On the other hatid.

sleep after the nig itmal laboratory < i not secm to bethe..^ A stronger influence i* Postponing sleep ditions of isolatio i that the morę sleep towards noon the ne> co mes, and when * Thus sleep durin* i terfered with, despite cally, should enhanc Similar obscrvatic 1 can select llieir ov | der conditions of lon (27,28). In the lat*—-that the factor mc turę lermination c • pcralurc cycle.

Sleepiness

Many qucstionna shift workers rep< i ers (29). Usually, luc I on the night shift, ha noon shift, and is In some studies «    ;

severe enough to hav falling asleep durinp The upper part tern of raled slee workers at a paper n afternoon shift sio*-^:i was Iow during th< i levcl at bedtime. sleepiness inereased di nouneed pcak duri This pattern of c demonstrated in i..— Physiological cvid* morę scarce. HowB urc 2, clectroencepBl were also carried off the aid of smali subjo for a duration of morning, afternot of figurę 2 shows tne ing the night shift ( of sleep can be se day sleep of little s 45-min nap during U sleep occurred fo^r subjects. Usually t • of the night shift other shift. Imporian doncd by the cor~‘ awareness that sl

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