What is the passage mainly about?

Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions from 29 to 33.

Today we hear more and more about the importance of getting enough sleep. Sleep can give energy to both our bodies and our brains. It can also affect our feelings, behaviour, and memory. Studies have shown that people in some countries spend less time sleeping and more time working. But our bodies cannot work well without enough sleep. Losing just one or two hours of sleep a night, over a long period of time, can badly affect our health. We cannot work for many hours; we can get angry easily; and we can even lose our IQ points. That explains why, without enough sleep, an intelligent person may have difficulty doing daily tasks.
Studies have also shown that the time of year seems to affect how much sleep we need. People usually sleep longer in the winter, sometimes as much as 14 hours a night. However, in the summer, they sometimes sleep as little as six hours, without having any problems.
(Adapted from Strategic Reading)