Sleep and Biological Clocks

Here are the questions (please answer both):

If you had to pick only one theory on why we sleep, which one would it be? Why do you pick this one out of the rest? Which line of reasoning or research finding do you think is the most compelling?
What is the relationship between sleep and our biological clock? What are some ways that you (or someone else) might disrupt your biological clock, even with day to day activities? What could you do to counteract this effect?