Figure 2

Limitations in dim light melatonin onset (DLMO)-based phase estimation. Curves schematically indicate the diurnal rhythms of melatonin concentrations. A solid curve and dashed curves represent a subject who was appropriate (a, control) and subjects who were inappropriate (b–e) for DLMO-based phase estimations. The red dashed line shows the threshold concentration for the determination of DLMO. The circadian phase in a, b, d and e is the same, although their calculated DLMOs are not the same. (a) A control example showing that the DLMO method works well with a dim light condition for a subject whose melatonin concentration is sufficient for a reliable determination of DLMO. (b) An example showing that the DLMO method does not work well because the melatonin concentration rapidly decreases in response to a transient light exposure because of a poor dim light condition. (c) An example showing that the duration of specimen collection under a dim light condition must be extended to more than 5–6 h because a rough prediction of DLMO is difficult because of the subject's irregular sleep–wake habits. (d) An example showing that phase estimation does not work well because the melatonin secretion levels differ markedly between individuals. (e) An example showing that the DLMO method does not work at all because the melatonin secretion dramatically decreases because of aging and neurodegenerative disorders. The DLMO-based method has limitations with respect to the interindividual comparison of circadian phases. A full color version of this figure is available at the Hypertension Research journal online.