7578409642

I2S8

THE NEW ENCLAND JOUKNAI. OK MKOICINK

May 3, IWO

nplalioii co an iiivcrsion of llic slccp-wakc schedule is ConsistctK willi llic finding lliat travclers adapt morc rapidly lo a liinc-y.onc sliifl if tlicy rcmain ouidoors on arrival ratlicr (lian in ilicir lioicl rooms°^,w and willi rcccnl studies suggcsiing tliai briglic liglil is an cffcc-l»vc syiichroniy.cr of circadian rliythinicily in liumnn subjccis/^,*^4?‘^w Our fitidings arc coin pa rabie willi tlić resuhs of cxpcrimcnis conductcd iindcr conditious of 24-bour dayliglit iii llic Aretie, in which adaptation of ihe body-temperaturę cycle to a rcvcrsal of llic accivi-ty—slccp sclicdulc was acliicvcd after only tlircc to four days among subjccts wlio concurrently rcverscd tlicir exposurc 10 tbc natural light-dark cycle by using blindfolds during daytimc sleep.* Howcvcr, it should be noted that llic resetting cffect of liglil on llic circa-dian paccmaker is critically dependent on ihe timing, intensity, and duration of the liglil exposure. The dc-sired phasc shifc may not be achieved evcn with the same amount of exposure to bright light** unlcss it is administered at appropriate limes.*⁹

The use of the constaru-routinc procedurę allowed the extcnt of adaptation of both physiologic and be-haviorai variablcs to be evaluatcd in the ćontrol and treatment studies. The endogenous circadian rhythms of body teinperaturc, plasma cortisol concentration, and urinary cxcretion ratę all failed to adjust to the Schedule of night work in the control studies, yet in the treatment studies they werc all elTeciively synchro-nized with the night-work schedule. The indexes of alertness and performance remained at their lowcst daily lcvcls from midnight to 08:00 hours in the control studies, cven after a week of nighttime work. In the treatment studies, these same indcxes during the night were significandy improved, and thcrc was a marked decline in alertness (or an inerease in sleepi-ness) during the daytimc, when night workers must attempt to sleep. This inerease in daytimc sleepiness is consistcnt with the shifc in phase of the physiologic variables and may account for the significandy in-Creascd sleep time among the men in the treatment studies. Taken together, these data suggest that the schedule of light and darkness to which these men werc exposed shifted a master circadian pace-maker that drives all these physiologic and bchavioral rhythms.

Misalignment of the circadian phase and slccp dep-rivation are the principal factors contributing to llic dccrcments in performance and inereased accident rates associated with night-shift work ⁹ Thercforc, the ability of cxposurc to light and darkness to adjust the circadian phasc and improvc the slccp of night-shift workers could liavc important implications for both industrial productivity and safety. Furthcrmorc, sińce circadian-pliasc misalignment, slccp dcprivatiou, or both may add to the dclctcrious conscqucnccs for hcalth that arc associated willi niglmimc work (soch as digcsiivc, cardiovascular, and slccp disorders), llic ability to inducc physiologic adaptation to sucli a schedule could also l»avc importaui conscquc«iccs for the hcalth of night workers.

Allliotigli tliis study is a step loward the dcvclop-ment of a practical treatment for inaladaptaiiou lo nighttime work, a numl>cr of important qucstions rcmain to bc answcrcd. They coitccrn tlić rclativc iui-(>oriancc of cx|M»surc to bright liglil during niglmimc work as coniparcd with darkness during daytimc slccp, vari:ibility butli hetwcen individuals and nc-cording to age and scx in llic pbasc-sliifting response to bright light, and the iiumhcr, duration, and itttcnsi-ty of liglil cx|x>surcs nccdcd to inducc and maintain optimal physiologic adaptation to nighttime work.

We are iodchtcd to tlić voluntccr subjccts in tlić Siudy, to the student research icdmicians, to Mr. J.W.Jcwctt for reoruitment of subjccts, to Ms. J. Swain and Ms. M. McCulfougli for tlić mctabolic dicts, to Ms. B. Potter and Ms. J. Norwood for hormonal assays, to Ms. M. McLcfUn for data processing, to Mr. A.E. Ward for the illustrations, and to Or. G.H. Williams for ovcra!l support.

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