The circadian clock is modulated by the timing of ingestion or food composition, but the effects of specific nutrients are poorly understood.We aimed to identify the amino acids that modulate the circadian clock and reset the light-induced circadian phase in mice and humans.Male CBA/N mice were orally administered 1 of 20 l-amino acids, and the circadian and light-induced phase shifts of wheel-running activity were analyzed. Antagonists of several neurotransmitter pathways were injected before l-serine administration, and light-induced phase shifts were analyzed. In addition, the effect of l-serine on the light-induced phase advance was investigated in healthy male students (mean ± SD age 22.2 ± 1.8 y) by using dim-light melatonin onset (DLMO) determined by saliva samples as an index of the circadian phase.l-Serine administration enhanced light-induced phase shifts in mice (1.86-fold;< 0.05). Both l-serine and its metabolite d-serine, a coagonist of-methyl-d-aspartic acid (NMDA) receptors, exerted this effect, but d-serine concentrations in the hypothalamus did not increase after l-serine administration. The effect of l-serine was blocked by picrotoxin, an antagonist of γ-aminobutyric acid A receptors, but not by MK801, an antagonist of NMDA receptors. l-Serine administration altered the long-term expression patterns of clock genes in the suprachiasmatic nuclei. After advancing the light-dark cycle by 6 h, l-serine administration slightly accelerated re-entrainment to the shifted cycle. In humans, l-serine ingestion before bedtime induced significantly larger phase advances of DLMO after bright-light exposure during the morning (means ± SEMs-l-serine: 25.9 ± 6.6 min; placebo: 12.1 ± 7.0 min;< 0.05).These results suggest that l-serine enhances light-induced phase resetting in mice and humans, and it may be useful for treating circadian disturbances. Background: Objective: Methods: Results: Conclusion: P N P
