The human body is constantly using energy, a process known as metabolism, which converts the calories from food into fuel. This energy is required to power every single function, from the beating of the heart to the firing of neurons in the brain. The amount of energy burned, or energy expenditure, changes throughout the day depending on a person’s activity level and physiological state. The body burns calories even during periods of complete rest to maintain life, but the rate at which this happens is not uniform. The biggest difference in energy use occurs when comparing the hours spent awake versus the hours spent asleep.
Calorie Burn While Awake: The Active Component
The energy spent simply existing while awake is measured as the Resting Metabolic Rate (RMR), which represents the number of calories burned in a day while stationary. This RMR is generally about 10% higher than the Basal Metabolic Rate (BMR), a theoretical measure of energy required for basic survival. RMR includes minor energy costs for processes like sitting up, talking, or recent food digestion, and typically accounts for 60 to 75 percent of the total daily energy expenditure.
Beyond the RMR, the waking state is defined by its highly variable and significant energy components related to movement. One major component is Non-Exercise Activity Thermogenesis (NEAT), which includes all the calories burned from movements that are not formal exercise. Actions like fidgeting, standing, walking around the office, or even minor household chores all contribute to NEAT. This constant, low-level activity provides a sustained elevation in calorie burn that is absent during sleep.
The other major variable is Exercise Activity Thermogenesis (EAT), which covers the energy expended during structured physical activity, such as running, weightlifting, or playing a sport. EAT can dramatically increase the metabolic rate for the duration of the activity and for a short time afterward. The combined effect of RMR, NEAT, and EAT makes the period of wakefulness the most metabolically expensive part of the day.
Calorie Burn During Sleep: Minimal Maintenance
The metabolic rate during sleep is closely aligned with the Basal Metabolic Rate (BMR), the minimum energy required to keep the body functioning. This energy is strictly dedicated to life-sustaining processes like breathing, circulating blood, maintaining body temperature, and cellular repair. To accurately measure BMR, a person must be completely rested, having fasted for at least 12 hours, and in a thermally neutral environment.
During a typical sleep cycle, the body’s energy expenditure is low and relatively stable overall. The metabolic rate can drop by approximately 15 percent below the RMR during the deepest stages of non-REM sleep. Conversely, during periods of Rapid Eye Movement (REM) sleep, which is characterized by intense brain activity, the metabolic rate slightly increases.
This slight increase during REM sleep is due to the brain consuming glucose for energy, accounting for about 20 percent of the calories used while at rest. Despite these minor fluctuations, the overall caloric burn during sleep is minimized because the body lacks the variable energy demands of movement, digestion, and conscious thought. As a simple estimate, many professionals use a figure of about 85 percent of a person’s BMR to approximate the hourly calorie burn during sleep.
The Direct Comparison: Answering the Core Question
The fundamental answer to the core question is that a person burns significantly more calories when awake than when sleeping. This difference stems from the cumulative effect of bodily functions and movements that cease or slow down during sleep. The resting state while awake, measured by the RMR, already includes minor energy expenditures that are excluded from the BMR, which approximates the sleep state.
The primary factor driving the higher energy expenditure while awake is physical activity, encompassing both formal exercise and unconscious movements like fidgeting or standing. Even when sedentary, the wakeful state maintains a higher baseline energy cost. The simple act of sitting upright versus lying down requires more energy expenditure.
If a person were to lie still in bed for eight hours while awake, their calorie burn would still be higher than if they were sleeping for the same duration. The body does not achieve the ultra-low metabolic state required for BMR unless a person is actually asleep. Therefore, the wakeful state, even when sedentary, always maintains a higher baseline energy cost.
Physiological Variables Influencing Energy Use
Multiple biological factors modulate a person’s metabolic rate in both the awake and asleep states, independent of their activity level. Body composition is a major determinant, as muscle tissue requires significantly more energy to maintain at rest compared to fat tissue. Individuals with a greater percentage of lean muscle mass will naturally have a higher RMR and BMR.
Body size and weight also play a role, as larger bodies require more energy to perform basic functions like circulating blood and moving, resulting in a higher absolute metabolic rate. Age is another influential factor, with the metabolic rate generally decreasing over time, partly due to the natural loss of muscle mass. Hormonal and neurological changes associated with aging also contribute to this decline.
Environmental temperature affects energy expenditure because the body must work harder to maintain its core temperature. When exposed to cold, the body expends energy to generate heat; in hot conditions, it expends energy through sweating and other cooling mechanisms. These thermoregulatory processes increase the metabolic rate in both the waking and sleeping states.