Astrangethinghappenstonearlyeverybodyatnight．Theyturnoffthelights，pullupthecoversandclosetheireyes．Sixorsevensleepinghourslater，theywakeupagain．Strange，isn'tit？
Sleeppuzzlesscience．Scientistsanddoctorswouldrathertalkaboutwhyonecan'tfallasleep．Theyarenotsurewhatcausessleep．Thebestconditionsforsleeparegoodhealthandmealneithertoobignortoosmall．Noworriesandacomfortableplaceareimportant，too．Theyadviseagainsttwoinbed．
Strangethingshappenduringsleep．Forexample，theymoveoften．Youwouldfeeltiredifyoudidn't．Youalsodream．Partofyourbrainisstillawakewhenyoudream．Dreaminghappenswhenthememoryandimaginationinpartsofyourbrainarestillawake．Experimentshaveshownmostofusdreamincolor．Baddreamsmaybecausedbystomachache．Don'tworryifyoudream．Somegreatstoriesandpoemswerebegunwhiletheirwritersslept．
45．Sleeppuzzlesscientistsbecausetheyarenotcertain___．
A．whyonecan'tfallasleep
B．whyonewakesup
C．bothAandB
D．whatcausessleep
46．Thebestconditionsforsleepare___．
A．goodhealth
B．amealneithertoobignortoosmall
C．noworries
D．goodhealth，limitedmeal，noworries and a good place
47．Youwouldfeeltiredduringsleepifyou___．
A．movedtoooften
B．sleptonyourback
C．didn'tmove
D．wenttosleeplate
48．Thebesttitleforthepassageis___．
A．HowCanYouGetaGoodSleep
B．TheStrangenessofSleep
C．WhatHappensDuringSleep
D．TheCausesofDream．