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.