Within-subject rhythmicity and stability of hunger, satiety, and physiological markers: insights from a five-day laboratory study in time-isolation conditions

Hunger and satiety are interoceptions involved in the control of food intake. How they change across the day and from one day to the next, as well as their relationships with physiological parameters, remain poorly understood. In a five-day laboratory study, 20 healthy male participants (24.2 +/- 3.3 years) were given 3 meals per day (breakfast, lunch, dinner), at times based on each individual's internal time. Subjects had a 8-h sleep opportunity at night recorded by polysomnography, during which time they were exposed to 4 different light intensities (0, 3, 8, or 20 lux). Hunger and satiety were assessed before and after each meal. Heart rate was measured continuously and glucose levels were sampled every 15 minutes. We found that: 1) hunger and satiety varied dynamically with the lowest levels at breakfast and the highest at lunch and dinner (p < 0.001 ); 2) Hunger and satiety were subject-dependent, and keep stable across the five experimental days; 3) heart rate was higher after than before meals (p < 0.0001), and higher around breakfast than dinner; 4) Glycemia before and after meals was respectively stable throughout the day and over the five-day study; 5) interoception was correlated with fluctuations in heart rate, glucose levels and sleep duration. Overall, our results reveal that hunger and satiety perception are individual characteristics. To better understand the role of interoception in food intake control, both hunger and satiety should be measured separately in their respective states.