I do that all the time, just leave the leaves in the gaiwan and hit them the next morning. They do lose a bit of flavor, but sometimes it's just the ticket. The 2 hour rule is just nonsense for oolongs and especially if you are brewing them with boiling water. I sometimes drink the same oolong all day long, with 4-5 hour breaks between sessions. A good oolong will go 10+ brewings. Puerh is even more lasting.