The body’s metabolism converts energy from food into the power needed for all functions, from breathing to exercising. Even during rest, the body constantly burns calories to maintain its most basic functions, a rate known as the Basal Metabolic Rate (BMR). Ambient temperature strongly influences BMR because the body must expend energy to maintain a stable core temperature. When the surrounding air cools, the body initiates mechanisms to increase heat production, which directly raises the rate of calorie burning, even during sleep.
The Body’s Mechanisms for Generating Heat During Rest
The body increases caloric burn in a cool environment through Non-Shivering Thermogenesis (NST). This heat generation occurs without the involuntary muscle contractions of shivering, which is a far less efficient way to stay warm. The main site for this specialized heat production in adult humans is Brown Adipose Tissue (BAT), a type of fat that functions like an internal furnace.
BAT is primarily located in the neck, collarbone area, and upper chest, converting chemical energy directly into thermal energy. When nerve signals sense a temperature drop, they activate the brown fat cells. These cells are packed with mitochondria and contain a unique protein called uncoupling protein 1 (UCP1) that short-circuits the normal energy-making process.
Instead of creating adenosine triphosphate (ATP) for cellular work, UCP1 allows energy from the breakdown of fuel, such as fatty acids, to be released immediately as heat. This mechanism forces the body to pull energy from its stores, increasing the overall metabolic rate during rest. Prolonged, mild cold exposure can also “recruit” more brown fat, developing a greater capacity for NST over time.
Quantifying Energy Expenditure in Cooler Sleep Environments
Scientific studies have attempted to measure the extra calories burned when sleeping in cooler temperatures compared to a thermoneutral environment. This increase, known as cold-induced thermogenesis (CIT), is typically measured as a percentage rise above the BMR. Research indicates that the metabolic increase from mild cold exposure, which avoids shivering, generally ranges from 3% to 7% above the resting metabolic rate.
One study found that men who slept in rooms cooled to 66°F (19°C) for a month saw a 42% increase in brown fat volume and a 10% increase in fat metabolic activity. This suggests a long-term adaptation, making the body more efficient at burning calories for heat. The specific temperature range of 60°F to 66°F (15.5°C to 19°C) is often cited as the “sweet spot” where the body initiates NST without causing uncomfortable shivering.
This additional energy expenditure translates to a variable, but measurable, number of extra calories burned during an eight-hour sleep cycle. A person sleeping in a 66°F room might burn an estimated few dozen to 100 or 200 additional calories per night compared to a warmer room, depending on their metabolic response.
Variables Influencing Cold-Induced Metabolic Rate
The actual rate of calorie burning in a cool environment is not uniform, as several biological variables modulate the metabolic response. Body composition plays a significant role, since body fat acts as insulation that slows heat loss. Individuals with less body fat tend to have a warmer lower critical temperature, meaning their metabolism increases at a slightly warmer ambient temperature.
Individual acclimatization, or how accustomed a person is to the cold, also changes the metabolic rate. Chronic exposure to cool temperatures enhances the body’s capacity for non-shivering thermogenesis, training the brown fat to be more active. Factors like height and biological sex also influence cold-induced energy expenditure.
The duration of cold exposure is a factor because the body’s response is dynamic. While acute exposure immediately activates existing brown fat, prolonged exposure over weeks increases the total amount of active brown fat tissue. This long-term recruitment results in a greater sustained capacity for calorie burning.
Realistic Expectations for Using Cold Sleep
While science confirms that sleeping cold increases calorie expenditure, it is important to maintain a realistic perspective on this effect. The increase in calories burned is often marginal for overall weight management. The extra energy expenditure of a few dozen to a hundred calories per night is not a substitute for dietary changes or physical activity.
The value of sleeping in a cooler environment may lie in its potential to improve metabolic health over time. Consistent cool exposure enhances brown fat activity, which is linked to better glucose metabolism and improved insulin sensitivity. However, if the temperature is too low and causes uncomfortable shivering, it disrupts sleep quality. The ideal temperature is one that stimulates NST without compromising deep, restful sleep, as good sleep is linked to better metabolic regulation.