The human body is constantly converting energy from food into fuel to sustain life. This process, known as metabolism, never stops, continuing even when the body is completely at rest. The energy expended to maintain basic functions during sleep is lower than during the day, but it remains a measurable part of your overall daily calorie usage. Understanding this fundamental rate of energy expenditure helps determine how many calories your body burns while you are asleep.
The Core Concept: Basal Metabolic Rate
The foundation for understanding calorie burn during sleep rests on the Basal Metabolic Rate (BMR). This rate represents the minimum energy required to keep your body functioning at rest. BMR accounts for energy needed for involuntary processes, such as heart contraction, breathing, and continuous cell repair.
BMR is traditionally measured under strict, controlled laboratory conditions, requiring a person to be fasted and at complete physical and psychological rest. A similar and often more practical measurement is the Resting Metabolic Rate (RMR). RMR is typically measured under less restrictive conditions and is slightly higher than BMR, but the two terms are often used interchangeably to estimate calories burned while inactive.
During the night, your body channels energy into several non-stop biological activities. The brain remains a significant energy consumer, especially as it consolidates memories and cycles through sleep stages. Energy is also needed for maintaining a consistent body temperature and for the continuous operation of organs like the liver and kidneys. These processes confirm that rest is a period of active, though minimal, energy expenditure.
Estimating Calorie Expenditure During Sleep
While the body is at rest, the calorie burn rate is closely linked to basal energy needs. For an average adult, this expenditure typically falls between 40 to 75 calories per hour. For instance, a person weighing 150 pounds might burn around 68 calories per hour, leading to hundreds of calories burned over a standard eight-hour night.
To estimate this nocturnal calorie burn, health professionals often first calculate the RMR using predictive formulas, such as the Mifflin-St Jeor or Harris-Benedict equations. These equations use your age, gender, height, and weight to provide a daily estimate of your resting energy needs. Since actual sleep is a state of even deeper rest than the conditions used to measure RMR, a slight downward adjustment must be made.
The metabolic rate during sleep (the sleeping metabolic rate) is generally 5% to 15% lower than the calculated RMR. For a simplified estimate, take your predicted daily RMR, divide it by 24 for an hourly rate, and then multiply that number by 0.90 to account for the reduction. For example, if a person’s daily RMR is 1,680 calories, the hourly rate is 70 calories; applying the 10% reduction suggests an hourly sleep burn of about 63 calories.
This calculation provides a reasonable approximation, which can then be multiplied by the total hours of sleep to find the total overnight expenditure. While these equations offer a helpful starting point, they only provide an estimate. They do not account for the continuous physiological variations that occur throughout the night, meaning the actual number of calories burned is a dynamic figure subject to several individual factors.
Factors That Influence Nocturnal Calorie Use
The total number of calories burned while sleeping is highly individualized and is significantly influenced by a person’s body composition and overall mass. Individuals with a greater overall body mass require more energy to sustain their basic functions, leading to a higher hourly calorie burn. Furthermore, muscle tissue is more metabolically active than fat tissue, meaning a person with higher muscle mass will typically burn more calories even when completely still.
Calorie expenditure also fluctuates as the body moves through the different sleep stages. Energy use decreases slightly during the deeper, non-rapid eye movement (NREM) stages, when heart rate and respiration slow down. Conversely, energy expenditure increases during rapid eye movement (REM) sleep, which is characterized by heightened brain activity and a higher rate of glucose metabolism, occasionally approaching the levels seen during wakefulness.
The environment in which you sleep also plays a modifying role in energy use. Sleeping in a cooler room, for example, may cause the body to expend extra calories through thermogenesis to maintain its core temperature. Genetic factors, age, and gender also contribute to an individual’s overall metabolic rate. Men generally have a higher BMR than women due to differences in average body size and muscle mass, and the metabolic rate naturally declines as a person ages.