Brief exposure to cool conditions activates brown adipose tissue, a specialized fat that burns calories to generate heat. Researchers from Leiden University in the Netherlands discovered that how well this calorie burn works depends on the time of day. The findings were shared with attendees at a European obesity research conference.
Brown fat, also known as brown adipose tissue, is a distinct type of fat that kicks in when the body is chilly. Its main function is to help keep core temperature stable by producing heat, a process that consumes energy and draws calories from stored fat.
As a result, cooling in the morning could be more effective for jump‑starting metabolism and fat loss in men, while in women the effect appears to differ. In this study, the strongest metabolic response was observed in the evening for some participants, suggesting a clear daily pattern in brown fat activity.
In a randomized crossover design, the study enrolled 24 adults, split evenly by sex. Each participant underwent a 2.5‑hour personalized cooling session, first in the morning around 7:45 and then in the evening around 19:45.
Water‑filled cooling mattresses were used, with temperatures gradually lowered until participants began shivering or a floor temperature of 9°C was reached. After the initial cooling phase, a continuous 90‑minute exposure followed. Throughout this period, researchers tracked energy expenditure and skin temperature at four timepoints to gauge brown fat activity.
Analyses showed that in men, both the energy burn tied to cold exposure and the skin temperature associated with brown fat activity were higher in the morning compared with the evening. In women, there was no significant difference in energy expenditure or skin temperature between the two time blocks, though women tended to tolerate the cold better in the morning. These results imply that evening cooling may offer a stronger overall metabolic benefit for some individuals, while morning sessions could yield greater effects for others, particularly men.
The study highlights how circadian biology can shape how the body responds to cold and how brown fat contributes to daily energy balance. For people in North America considering cold exposure as a tool for metabolism or weight management, timing may be as important as the cold itself. The findings invite further research into how age, body composition, and lifestyle interact with the morning versus evening responses of brown fat. n