You use energy no matter what you're doing, even when sleeping. The BMR Calculator will give you an estimate of the number of calories you'd burn if you stayed in bed all day. Decreased BMR means that you burn fewer calories!
BMR (Basal Metabolic Rate)
BMR decreases as you age so you’ll notice that every year, it becomes harder to eat whatever you want and stay slim Likewise, depriving yourself of food in hopes of losing weight also decreases your BMR. However, a regular routine of cardiovascular exercise can increase your BMR, improving our health and fitness when your body’s ability to burn energy gradually slows down.
The only factor BMR does not account for is lean body mass, therefore what ratio of muscle-to-fat the body has. Remember, leaner bodies need more calories, for the super muscular it will underestimate calorie needs and for the obese it will overestimate calorie needs.
The American College of Sports Medicine (ACSM) recommends that calorie levels never drop below 1200 calories per day for women or 1800 calories per day for men. Even these calorie levels are quite low.
If you do have any concerns about your health and weight, please consult your Doctor or specialist for a health & fitness assessment before starting any exercise & weight loss program.