Most Fitness/Activity bands 'attempt' to calculate all the calories your body uses per day - not just from exercises. From midnight (when my old FitBit resets), it used to say I had burned around 450kcal when I woke around 7am.
All this is depending on you inputing correct physical metrics i.e. Weight, Height, Age, etc. All similar kinds of band/monitor will always give 'estimates'. For more accurate calorie burn, or fitness monitoring, you need to look elsewhere, such as Polar, Garmin, etc.