help me to correct my sentences!

1.Doctors say that we should sleep at least eight hours a day, but many of us ignore this advice.

2.Some students sleep for only about six hours but it is not enough.

3.Lack of enough sleeping has negative effect to students.

4.They will find it difficult to concentrate in class and their health will be affected.

5.Many teenagers complain about the amount of homework and examination they have .

6.But I think these are not excuse for sleeping late.

7.Instead of watching television, surfing the internet or playing computer games, they should tackle their homework early and try to go to bed at a reasonable hour.

2008-04-24 23:56:18 補充：
7. Instead of............. at a reasonable time.

All correct except point 7 :

7. ............they should work on their homework early and try to go to bed at a reasonable time.