You are not alone As is often the case，you are trying hard to keep your eyes open in class after a late night doing piles and piles of homework. "（1）

B

B

" You are probably thinking， "This weekend I'll make up for all the sleep I have lost."
   （2）

A

A

Most people think the same way—if you stay in bed longer on Saturdays or Sundays，you will recover your missed sleep during the week.
   However，this is not correct.（3）

D

D

Actually，researchers have just discovered that sleeping during the weekend will not do you any good，but rather make you feel more tired on Monday morning.
   Our body keeps a clock inside.It tells us when to wake up in the morning and when to feel sleepy at night.But the time on our body clock can be changed once we start taking on different sleeping habits.（4）

G

G

This may mean on Monday morning we sometimes struggle to get out of bed.
   If we decide to get up only 20 minutes or half an hour later，it won't matter.This is because our body clock can accommodate（適應(yīng)）to a delay of up to an hour，which means that a short lie-in at weekends is unlikely to have any big effect.We needn't worry about it at all.（5）

F

F

   So，what should we do to balance and make up for all the sleep we've missed？Scientists say that simply eight hours of sleep would do the trick，not any longer.

A.You are not alone.
B.I need more sleep!
C.Sleep is important for everyone.
D.Sleeping is nothing like paying back money.
E.However，if we sleep longer，our sleep becomes less efficient.
F.But delays of up to two hours or more can throw our body out of order.
G.This is why staying in bed for longer than usual might confuse our body clock.