As for the powder itself, heat is the number one enemy of gunpowder, humidity is number two. Powder ages and heat accelerates the deterioration of gunpowder. Sunlight is not good either so keep your gunpowder in the dark. Sunlight is bad on all plastics and rubbers.
What he said,please return powder after each reloading session to the opaque container, power will degrade over time and there is no such thing as overnight which sometimes turns into weeks, plus the safety factor of using wrong powder at next reloading session.