Coffee is a wonderful delicious beverage anytime of the day. Thats why you should be able to enjoy a cup of decaf during the night hours even if your ready for bed.
Decaffinated coffee is not 100% caffeine free. A decaf coffee only needs to be 97% caffeine free according to the USDA. That means that an average 12oz cup of decaffeinated coffee, which originally has about 180mg of caffeine, now would contain about 5.4mg of caffeine.
Colombian Decaf Swiss Water Process