A good sleep

This afternoon I asked Class 310 what is most important to them.  Many of them said having friends.  A student answered, "Sleep."  I remember when I asked my former students what they wanted to do most, many of them would answer, "Sleep."   It seems that generally speaking, high school students in Taipei don't get enough sleep.  Is it because they have so much to do--homework, preparation for countless tests, cram school to attend, etc.--or because they don't manage their time well?