External validation of the Japanese version of the reduced morningness-eveningness questionnaire (rMEQ) score using dim light melatonin onset and sleep-wake behavior in young adults.

The Reduced Morningness-Eveningness Questionnaire (rMEQ), a five-item version of the 19-item MEQ, is a practical tool for assessing circadian typology or "morningness" and "eveningness" preference. Although we previously validated the Japanese version of rMEQ with MEQ, external validation against an established standard was lacking. This study aimed to additionally validate the Japanese rMEQ in young adults by assessing the dim light melatonin onset (DLMO), a marker of circadian phase, and subjective/objective sleep-wake habits. Twenty healthy young adults (mean age: 23.0 ± 1.9 years) participated in this study and were assessed the circadian typology by rMEQ and MEQ. The sleep-wake habits were measured by sleep diary, actigraphy and the Pittsburgh Sleep Quality Index (PSQI). Salivary melatonin samples were collected to determine DLMO. Results showed significant correlations between rMEQ scores and DLMO, as well as sleep variables based on sleep diary, actigraphy and PSQI. Correlations between rMEQ scores and these variables were consistently stronger than those for MEQ. These findings highlight the validity of the Japanese rMEQ and that the rMEQ more accurately reflects circadian typology and internal circadian phases compared to MEQ, at least in Japanese young adults. The Japanese rMEQ could be a valuable tool for efficiently assessing circadian typologies in Japanese young adults.