When it feels like just staying awake takes an effort, you are right. You burn calories just sitting around all day, and those calories comprise your resting metabolic rate. Any physical activity you perform adds to the calorie count.
A male's metabolism burns more calories per day than a female's metabolism because men have a higher percentage of lean muscle mass than women do. The metabolisms of both men and women slow down as they age, the Mayo Clinic explains.
The heavier you are, the more calories your metabolism burns to sustain itself. You activity level also affects how many calories your metabolism burns each day. When your exercise and build lean muscle mass, you increase your metabolism slightly, too, the Marquette General Health System notes.
Calories Per Day
You burn 2,000 calories per day if you are a 27-year-old woman who is 5 feet, 7 inches, weighs 135 pounds and exercises for about 30 minutes a day, the U.S. Department of Agriculture's MyPyramid website calculates. When you reach 55 years of age, you only burn 1,800 calories even if you keep with your exercise regimen. You burn 2,600 calories per day as a 27-year-old man who is 5-feet-11, weighs 170 lbs. and exercises 30 minutes per day. When you reach the age of 55, you only burn 2,400 calories per day.