I did a quick search and found this post:
Re: Leaving leaves for extended time during gongfu
New postby britt » Sep 25th, '10, 17:47
If I knew I was going out, I'd just brew some the conventional way and dump the leaves before I left. If not, I wouldn't hesitate to use the same ones when I returned. I have left oolong leaves in an Yixing overnight and they've been fine to use as-is the next morning. Japanese tea would be dumped and new leaves added, but oolongs seem to be very forgiving in this regard.
I have had the same experience. On many occasions there is just not enough time or my wife gets "teaed out." I do not want to throw out a great Oolong that has only been steeped 4-6 times.
Some great Oolongs (Phoenix and Wuyi) do well for me staying overnight (5 to 6 grams of tea in a 100ml yixing.) I swear that on the second day the tea on initial steeping tastes very good, better than the last steeping from day 1. It seems the stems seep out more flavor overnight. That flavor is then drawn out with that initial 2nd day of steeping. I have to admit to doing this with a tea in the pot for about a week with steeping every day.
I would not recommend it with the "greener" teas like sencha and light Taiwanese Oolongs.
Leaving leaves in water all night seems strange unless you were at the last steeping trying to wring out the last bit of flavor.
It seems a number of people do this. Maybe it would be a good topic for a poll.