If you live in a very high humidity environment then you won't want to leave it that way for very long (read many months) unless you take steps to keep the water out, and several ways of doing that have been pointed out already. Leaving the powder and ball charge in overnight after an unsuccessful hunt is perfectly alright, just be sure to un-cap the nipple for safety. The only thing you might have to worry about in case you leave your rifle that way for several weeks in very high humidity is the powder getting a little weak from absorbing water, but it's not going to invite rust and corrosion. Properly sealed, traditional gun powder in your rifle will last for many years, however, substitute powders get weaker over time, and it wouldn't be a good idea to rely on them if stored in your rifle for many months.
Hope this helps.