Why do people say get plenty of rest when you feel like your getting sick?

Whenever I do this, like when I have a cold. The day before when the cold is coming on and I feel all the drainage in my mouth, when I go to sleep and wake up the next morning not beeing able to taste or smell things and I sound like Ive been holding my nose the entire night. I did an experiment a few weeks ago when I felt like I was getting a cold. I stayed up the whole entire night and just drank orange juice and I kept my head forward to try and keep the drainage from going into my mouth constantly, and the cold went away. It seems like when you sleep, you let the drainage just take over so when you wake up, you have the cold, I don't know how it works, but it pretty much ONLY takes effect when you sleep. What do you think?