The iHealth Wave calculates all the calories burned throughout the day. Those burned by physical activity but also that burned by the basal metabolism.
The basal metabolism corresponds to the "incompressible" energy needs of the organism, it means the daily minimum energy expenditure enabling the body to function. It is determined by age, sex, height and weight. At rest, the body consumes energy to maintain its functions (heart, brain, lungs ...).
This is why when you wake up, even if you have not taken any steps, you will already see a certain amount of calories burned.
*credit to Sylvain of iHealth Labs EU