The college entrance examination is just around the corner. Homework and tests will keep many of you up late at night, and you may plan to make up for your lost sleep during the weekends.     1     is it useful? A study published in the journal Current Biology completely has changed     2     people used to think. It shows that the habit of sleeping in on weekends doesn’t fix the damage done by a lack of sleep during the week. Even     3     (bad), it may damage your health.