1. D。语义理解题。只要看到第2段第3句话They are not sure what causes sleep.，答案不言自明。
2. D。语义理解题。由第2段第四、五句话 The best conditions for sleep are good health and meal neither too big nor too small. No worries and a comfortable place are important, too可知答案。
3. C。语义理解题。由第3段第3句话 You would feel tired if you didn’t (move) 可知答案。
4. B。从文章意思来看，A项明显不符，C项内容片面，D项文中没有说明，所以应选B项。
