I started applying the Japanese Water Therapy. It suggests to drink about 1 liter of water right after waking up.
It makes intuitive sense to me.
The human body needs about 2-4 liters of water per day. As a heuristic, you can say it needs 100 ml per hour. So if you sleep for 8 hours, you might need 0.8 liters.
It also makes sense that it helps with flushing out toxins, and potentially preventing headaches.
I’ve been drinking 100-300 ml of water after waking up anyways, and now I’ve upped it to 1 liter.
[Thank you Anuj Didwania for the inspiration.]