Some "why" questions are causes, and others are actually not a causal essay, but rather a "how" explaining essay. Why people need to sleep falls into this category because there are some pretty definitive answers to this question that most authorities would agree about. A good causal analysis asks a question that people don't agree about. Here are a few on the topic of sleep:
What causes people to have insomnia?
Why is there an increasing trend in people not getting enough sleep?