Might depend on the climate, but I would think coke will absorb some moisture if left out overnight so it might not be as potent by weight if there's a bit of water in it. I believe cocaine can degrade a bit if left wet for too long, but one night shouldn't hurt. If there's a fan running or a heater on though, it may actually dry out a bit. I wouldn't know, lines of coke don't get left out for too long around here haha.
Either way, store it properly and it will be fine.