MakingLightof1 Sleep
Allwehaveaclocklocatedinsideourbrains. Similartoyourbedsidealarmclock， yourinternalclock2 runsona 24-hourcycle. Thiscycle，calledacircadianrhythm，helpscontrolwhen
youwake，whenyoueatandwhenyousleep.
Somewherearoundpuberty，somethinghappensinthetimingofthebiologicalclock. The
clockpushesforward，soadolescentsandteenagersareunabletofallasleepasearlyastheyusedto. Whenyourmothertellsyouitstimeforbed，yourbodymaybepushingyoutostayup3 forseveralhoursmore. AndthelightcomingfromyourcomputerscreenorTVcouldbepushingyoutostayupevenlater.
Thisshift4 isnaturalforteenagers. Butstayingupverylateandsleepinglatecangetyourbodysclockoutofsyncwiththecycleoflightanddark5. Itcanalsomakeithardtogetoutofbedinthemorningandmaybringotherproblems，too. Teenagersareputinakindofagraycloud6 whentheydontgetenoughsleep，saysMaryCarskadon，asleepresearcheratBrownUniversityinProvidence，RI7 .Itaffectstheirmoodandtheirabilitytothinkandlearn.
Butjustlikeyouralarmclock，yourinternalclockcanbereset. Infact，itautomaticallyresets
itselfeveryday. How? Byusingthelightitgetsthroughyoureyes.
Scientistshaveknownforalongtimethatthelightofdayandthedarkofnightplayimportantrolesinsettingourinternalclocks. Foryears，researchersthoughtthatthesignalsthatsynchronizethebodysclock8 werehandledthroughthesamepathwaysthatweusetosee.
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