This was achieved by having participants complete a 240-hour ‘forced desynchrony’ (FD) protocol in which the daily behaviors, sleep and meals occurred across all circadian phases (12 recurring 20-hour behavioral cycles, maintaining a 2:1 wake/sleep ratio), with constant dim light (<4 lux) to avoid resetting the phase of the circadian system. Thereafter, all behavioral and environmental factors that can potentially affect appetite or the circadian cycle (e.g., meals, sleep, activity, posture, room temperature, light) were controlled while hunger and appetite were measured across the circadian cycle.
Participants maintained a fixed sleep/wake schedule with 8 h in bed for 2–3 weeks at home then adapted to the laboratory environment for two days and two nights while maintaining the same 8-hour sleep schedule. All subjects gave written informed consent and the studies were approved by Partners Human Research Committee. Twelve healthy adults (6 male age, 20–42 y BMI, 19.9–29.6 kg/m 2) completed a 13-day in-laboratory protocol. To examine this third possibility that the internal circadian clock regulates hunger, we assessed the sensation of hunger and appetite in healthy participants throughout a 13-day in-laboratory protocol that controlled for the timing of meals and sleep.Īspects of this study testing different hypotheses have been published ( 4– 7).
This paradox could be explained by: (a) a large meal in the evening and decreased energy expenditure during sleep compensating for the prolonged overnight fast (b) sleep itself inhibiting appetite, with this inhibition carrying over into the morning wakefulness period and/or (c) an endogenous circadian influence on appetite that suppresses hunger during the morning. Indeed, in Western cultures, dinner is the largest meal of the day (35–36%) ( 3).
Otherwise, appetite would be greatest in the morning following the extended overnight fast, whereas humans typically are not hungriest in the morning, breakfast is usually the smallest meal of the day-accounting for only 16–18% of the daily calories, and many people skip breakfast altogether ( 3). Food intake does not appear to be simply regulated by a homeostatic energy balance mechanism. In addition to what we eat, there is evidence of the importance of when we eat in the development of obesity ( 1, 2, 18).