Generally: A bad idea. Refrigerating brewed coffee for a little while doesn’t hurt anything, but leaving it in the refrigerator overnight seems to increase it’s diuretic qualities four or five times over. I noticed a build up of oil on the top of the coffee when it sits in the refrigerator for extended periods, which may have something to do with the speedy way it goes through one’s digestive system.
As part of the opening shift at Starbucks we would make an extra strong batch of coffee, which we refrigerated for a single day of use. We would dispose of any left-over iced coffee rather than store it overnight, which seems to confirm my own findings, mentioned above.
There is another way to make chilled coffee, which is known as toddy. Toddy is iced coffee that is brewed cold (or at room temperature) from the beginning. This requires another kind of coffee maker (a toddy maker) that allows the coffee to steep for as long as 12 hours. Toddy is supposed to be less acidic and less caffeinated than regular coffee. It’s also supposed to be storable for several days.
I tried toddy once; it tasted slightly better than Starbucks’ coffee-concentrate poured over ice.