ANSWER: Your Fitbit tracker’s calories-burned tally resets each night at midnight. So the number you see on your tracker (or in your Fitbit app) first thing in the morning is your estimated calorie burn for the day so far.

It’s based largely on your basal metabolic rate (BMR) or the energy your body expends at rest—even when you’re sleeping!—just to keep you alive. BMR powers basic but vital life processes, like breathing and thinking, and accounts for most of your daily calorie burn. (For more information on BMR, see How Does Fitbit Estimate How Many Calories I’ve Burned?)

Once you’re up and moving around, Fitbit will also take your heart rate (if your tracker can measure that) and any activity—automatically or manually logged—into account.