Haveyoueverexperiencedthefeelingoffallingasyoustarttofallasleep？Sometimesyoumaycometorealizethisfeeling．Ifso，don'tworry．Manypeoplehavefeltthesamething．Infact，thesemuscletwitches（肌肉抽搐）inthearm

　　Haveyoueverexperiencedthefeelingoffallingasyoustarttofallasleep？Sometimesyoumaycometorealizethisfeeling．Ifso，don'tworry．Manypeoplehavefeltthesamething．

　　Infact，thesemuscletwitches （肌肉抽搐） inthearms，legsorotherpartsofthebodyarecalledhypnicjerks （入睡抽动） orsleepstarts．Andtheyareverycommon．Upto70percentofpeopleexperiencethemsometimes，HuffingtonPostreported．Butwhatcausesit？

　　Therearesometheories（理论）．Somescientistsbelievethatthefeelingoffallingcomesfromthebodyrelaxingitself，especiallyifthepersonisanxiousorunabletogetcomfortable．Asyou'reabouttosleep，yourmusclesbecomerelaxed．Butyourbrainstaysawake．Itmisunderstandstherelaxingofyourmusclesasasignthatyou'refalling．Thenthebrainjerksthebodyawakeasifit'stryingtoprotectyou．

　　Otherscientistsbelievethatdifferentphasesofsleepmightcausethehypnicjerks．Whenwesleep，weexperiencetwophases：non-rapideyemovement （NREM，非快速眼动睡眠期） and rapideyemovement （REM，快速眼动睡眠期）．DuringREM，ourheartrate （心率），breathingandnervoussystemcanactirregularly （不规律地）．Ifthebodyexperiencesflashes （一瞬间） ofREMwhileenteringsleep，theseirregularitiescouldcausetwitches．

　　Ifyoudon'twanttoexperiencehypnicjerks，scientistshavesomesuggestions．Whenyoubecomedeeplyrelaxed，youcanlearnhowtoreconnect （再联系） slowlybymovingyourfingersandtoesalittlebit．Thenstarttomoveyourheadandopenyoureyes，whilebeingawareofyourselfasyougetoutoftherelaxedstate．

　　Butexpertsagreethathypnicjerks，thoughsometimesscary（吓人的），areharmless．Ifitwakesyouup，simplyrolloverandgobacktosleep．Butkeepinmind：acaffeinated （含咖啡因的） drink，excitingeveningactivitiesorstressmaymakehypnicjerkshappenmoreoften．

　　39．Weknowfromthepassagethat___．

　　A．theyhappenaspeoplewakeup

　　B．muscle twitches are only inthearms and legs

　　C．theycancausepeopletofalloutofbed

　　D．morethanhalfofpeopleexperiencethemsometimes

　　40．Somescientistsbelievethatpeopleusuallyhavethefeelingoffallingwhen___．

　　A．theirbodytriestorelaxitself

　　B．theyfeelsafe

　　C．theirb