The Human Circadian System Has a Dominating Role in Causing the Morning/Evening Difference in Diet‐Induced Thermogenesis

Objective Diet‐induced thermogenesis (DIT) is lower in the evening and at night than in the morning. This may help explain why meal timing affects body weight regulation and why shift work is a risk factor for obesity. The separate effects of the endogenous circadian system—independent of behavioral cycles—and of circadian misalignment on DIT are unknown. Methods Thirteen healthy adults undertook a randomized crossover study with two 8‐day laboratory visits: three baseline days followed either by repeated simulated night shifts including 12‐h inverted behavioral cycles (circadian misalignment) or by recurring simulated day shifts (circadian alignment). DIT was determined for up to 114 min (hereafter referred to as “early DIT”) following identical meals given at 8AM and 8PM in both protocols. Results During baseline days, early DIT was 44% lower in the evening than morning. This was primarily explained by a circadian influence rather than any behavioral cycle effect; early DIT was 50% lower in the biological evening than biological morning, independent of behavioral cycle influences. Circadian misalignment had no overall effect on early DIT. Conclusions The circadian system plays a dominating role in the morning/evening difference in early DIT and may contribute to the effects of meal timing on body weight regulation.