Por que perdemos o sono?

The brain is like a group of people talking to each other. When you're watching TV, the part of your brain that watches TV says "Shut up guys, I'm watching TV," so you can focus without thinking about cake or math. As a result, the others sit silent, grow bored, and fall asleep, until only the TV watcher part of the brain is left. Left by himself, he too gets bored and falls asleep.

When you're in bed, assuming you aren't counting sheep or something, the entire brain is kind of in free time mode, and any part of the brain can speak up if it wants to. They start talking to each other, and even if one of them starts to drift to sleep, the others wake it up either by deliberately talking to the sleepyheads or just being noisy. Eventually more and more of the parts of the brain fall asleep from sheer exhaustion no matter how loud the others are, and eventually the last one passes out and you are asleep.