My grow room seems to be dropping to about 64% humidity and 63 degrees at night. I was curious whether this was too extreme for the long term?
Not really, 63 is pretty low but not TOO low. I think like 65-68 would be damn near perfect. Under 60 and I could see some possible issues. I doubt 64% would cause you any grief either. Now if its 63 ambiant but the plants sit on a concrete floor or something like that you need to concider the difference in temps on the ground vs the air or the roots could get to cold and shock. If the floor feels cold to the touch put something under the pots.
My bud room is about the same and the only thing I tend to notice is more of a fall color change in my leaves than I did during the summer with higher night temps. This may also be trait of the mystery bag seed I'm currently growing so your mileage may vary! I wouldn't let it go below 60 though, if you can help it...