Water was a vexing issue for our ancestors. It was heavy and awkward to carry - no convenient containers except... their stomachs. So, upon waking they took a big, long draft from the nearby stream before heading out to hunt for the day.
Like other primal behaviors, there is wisdom in emulating this practice. Our bodies are accustomed to the rhythm of sleep followed by an influx of fresh water. While sleeping, our body is busy at the cellular level putting all the trash out onto the street for collection. Taking in a good amount of water (about 20 oz) allows all these toxins to be flushed out of our system. If we don't do this, our cells have to drag all those stinky garbage bags back into the house and wait for another opportunity to get rid of them. Yuck.
Bodybuilders follow this practice as do the Japanese. I find that interesting because I respect both groups - one for their attitude of self-experimentation in finding things that just work, and the other for the wisdom of their traditions. So, wake up, drink water, and go.