Military personnel and athletes have a very high energy expenditure which is increased during certain key periods (intense training, competition and mission). Compensating for this expenditure through food can be complicated by physiological ingestive limits and logistical and organizational constraints (number of meals, availability of food), which leads these populations to regularly experience energy deficit situations (intake below requirements), which could alter physical and cognitive performance and major physiological functions.

Among the many constraints to which military personnel and athletes are exposed to (stress, sleep deprivation, travel, etc.) that can increase the risk of energy deficits, the impact of thermal environmental constraints is not well known. The seasonal impact and travel to countries with very different thermal environments can lead these populations to experience cold and hot conditions for long periods.

Understanding how heat and cold exposure modifies appetite and energy intake therefore appears to be of great importance.

The hypothesis of this study is that a 24 h heat exposure would produce a rapid and long-lasting anorexigenic action impacting energy intake, while a cold exposure would produce the opposite effect (orexigenic action).

Inclusion Criteria:

• Male
• Aged 18 to 40 years old
• Active (≥ 3 hours of physical activity per week)
• Healthy (no metabolic pathology)
• Affiliated to the social security system
• Having given written consent to participate.

Exclusion Criteria:

• Unusual eating pattern (< 3 meals per day) or irregular eating pattern
• Pattern of dietary restriction (determined by the Three-Factor Eating Questionnaire-21)
• Unusual sleep pattern (Epworth score > 10, insomnia severity score > 14, Pittsburgh Sleep Quality Index > 5), sleep duration < 6 h per night or difficulty falling asleep)
• Metabolic pathology
• On medication
• Previously exposed to unusual levels of heat or cold (vacation, missions) in the 4 months prior to inclusion
• Refusal to participate.

The study population will be composed of young and healthy men.