Scientists have uncovered a key insight into human metabolism: the calories burned at rest vary significantly throughout the day. Research shows people burn about 10% more calories in the late afternoon and early evening compared to early morning. These findings highlight the circadian clock's vital role in metabolic regulation and explain why irregular schedules from shift work or travel often lead to weight gain.
To pinpoint circadian effects on metabolism—isolated from activity, sleep-wake cycles, and diet—researchers studied seven participants in a fully controlled lab environment. With no clocks, windows, phones, or internet, subjects followed scheduled sleep times shifted four hours later each night, simulating daily westward travel across four time zones for four weeks.
Results indicated resting energy expenditure is lowest during the circadian phase ~0°, aligning with the core body temperature drop in late biological night. It peaks at ~180°, roughly 12 hours later, in the biological afternoon and evening.
Participants' respiratory quotient, indicating macronutrient use, also followed circadian patterns—lowest in the evening and highest in biological morning.
This landmark study provides the first human circadian profile of resting energy expenditure and respiratory quotient, decoupled from activity, sleep, and diet influences.
"It's not just what we eat, but when we eat—and rest—that influences how much energy we burn or store as fat," noted the lead researcher. "Regularity in eating and sleeping habits is crucial for metabolic health."
