Many people monitor their daily energy intake and output, leading to curiosity about how many calories are expended while sleeping. Even when the body is at rest, fundamental biological processes require a continuous supply of energy. This constant consumption is known as metabolism, and it never truly stops, ensuring that life-sustaining activities continue throughout the night.
The Science of Restful Energy Expenditure
The calories burned while sleeping are primarily a reflection of the body’s Basal Metabolic Rate (BMR) or Resting Metabolic Rate (RMR). These rates describe the energy required to fuel involuntary functions that maintain life, such as the operation of the heart and lungs, blood circulation, and cell repair.
BMR is measured under strict laboratory conditions after a full night’s sleep and an extended fast, representing the body’s minimum energy state. RMR is a more practical measurement, typically about ten percent higher than BMR. Since sleep is a state of near-complete rest, the energy used during this period closely aligns with the BMR.
Approximately 60 to 70% of the total daily calories burned are consumed by these basic metabolic processes. The brain alone consumes a significant portion of this energy, using glucose to sustain its activity throughout the entire eight hours of sleep.
Estimating the Sleep Calorie Burn
To estimate the calories burned during eight hours of sleep, one must first calculate their individual BMR. Formulas like the Mifflin-St Jeor or Harris-Benedict equations use factors such as age, sex, weight, and height to approximate this daily energy requirement. The 24-hour BMR is then divided by 24 to find the hourly rate, which is multiplied by the hours slept.
Metabolism during sleep is often slightly lower than the calculated BMR, typically by five to fifteen percent, due to decreased muscle tone and reduced overall activity. On average, most adults burn between 40 and 55 calories per hour while sleeping.
For a person weighing 150 pounds, this translates to burning an estimated 320 to 440 calories over a full eight-hour night of sleep. A heavier individual will have a higher BMR and consequently burn more calories. For example, a 200-pound person might burn 420 to 580 calories in the same period, illustrating the direct relationship between body mass and resting energy expenditure.
Variables That Adjust Nocturnal Calorie Use
The calculated BMR provides a baseline, but several physiological and environmental variables cause nocturnal calorie use to fluctuate.
Body Composition
Body composition significantly impacts resting energy needs. Muscle tissue is more metabolically active than fat tissue, even at rest. Individuals with a higher percentage of lean muscle mass naturally have a higher BMR and burn more calories while sleeping.
Environmental Temperature
The ambient temperature of the sleep environment also plays a role. Sleeping in a slightly cooler room can compel the body to expend more energy to maintain its core temperature. This process, known as thermogenesis, can lead to a minor increase in calorie burn.
Sleep Stages
Metabolic activity changes across the different stages of the sleep cycle. During deep, non-rapid eye movement (NREM) sleep, the metabolic rate drops to its lowest point. Conversely, during rapid eye movement (REM) sleep, the brain becomes highly active. This increased neural activity causes a temporary surge in glucose consumption, which slightly elevates the hourly calorie burn.