Passage Three
The amount of sleep a person needs tofunction effectively varies considerably from individual to individual and from time to time.Newborns spend an average of 16 hours a day sleeping.Teenagers may spendasmuchas10to11hoursasleepeachnight.Studentsingraduateschoolaverageeighthoursa night.Although theamountof sleepa personneeds may vary,itdoesappear thateveryonesleepsandthat sleep is crucial to the normal functioning of the body.
Have you ever noticed that there are certain times of the day when you are more alert or more tired?Peopleseemtohaveaninternalbiologicalclockthatregulatesthesleep-wakefulnesscycle.Bloodpressure,heartrate,andappetiteallfollowcircadianrhythms( 昼 夜 节 律 ) .Acircadianrhythmisabiological clock in our body that regulates the responses of our body within a time period of 24 or 25 hours.Circadianrhythmsoperateevenwhencluesfordayandnightareremoved .Forexample,we usually adapt our sleep patterns to the light of day and dark of night;yet researchers who have lived for months at a time in the depths ofa cave have still maintained a rhythm to their behaviours.Without anyenvironmentalclues,peoplemaintainedtheircircadianrhythmsonabouta24-to25-hourcycle.Researchers have determined that humans have a circadian cycle of approximately 24.18 hours.
Circadian rhythms do not control our sleep cycles;the environment and the 24-hour day control our cycles.Thus,whenyoumisssleep,thisdisruption( 打 乱 )becomesveryapparent.Sometravellersexperience jet lag(时差反应) .This occurs when their internalcircadian rhythms do not match the realclock time.Whatdo you do to cure jet lag?It usually takes about one day for each hour of time changeto return to your normal circadian clock.
题目:Where does the passage most likely come from?