So creatine adds the phosphates back to the adenosine? However, it's a good reason to exercise. Burn the phosphates from ATP and be left with adenosine, which as you say promotes sleep.

However, I don't think it is so much of a signal of your brain having an idea of how long you've been awake, that's the circadian cycle. I always looked at adenosine as a driver of sleep need. If you burned this much ATP, we're going to need to recover. Seems like an elegant process.