Calculating the Dim Light Melatonin Onset: The Impact of Threshold and Sampling Rate

Abstract

The dim light melatonin onset (DLMO) is the most reliable circadian phase marker in humans, but the cost of assaying samples is relatively high. Therefore, the authors examined differences between DLMOs calculated from hourly versus half-hourly sampling and differences between DLMOs calculated with two recommended thresholds (a fixed threshold of 3 pg/mL and a variable “3k” threshold equal to the mean plus two standard deviations of the first three low daytime points). The authors calculated these DLMOs from salivary dim light melatonin profiles collected from 122 individuals (64 women) at baseline. DLMOs derived from hourly sampling occurred on average only 6–8 min earlier than the DLMOs derived from half-hourly saliva sampling, and they were highly correlated with each other (r ≥ 0.89, p < .001). However, in up to 19% of cases the DLMO derived from hourly sampling was >30 min from the DLMO derived from half-hourly sampling. The 3 pg/mL threshold produced significantly less variable DLMOs than the 3k threshold. However, the 3k threshold was significantly lower than the 3 pg/mL threshold (p < .001). The DLMOs calculated with the 3k method were significantly earlier (by 22–24 min) than the DLMOs calculated with the 3 pg/mL threshold, regardless of sampling rate. These results suggest that in large research studies and clinical settings, the more affordable and practical option of hourly sampling is adequate for a reasonable estimate of circadian phase. Although the 3 pg/mL fixed threshold is less variable than the 3k threshold, it produces estimates of the DLMO that are further from the initial rise of melatonin. (Author correspondence: [email protected])

Keywords:

• Circadian
• Dim light melatonin onset
• Phase
• Sleep
• Threshold

ACKNOWLEDGMENTS

This work was made possible by grants from the National Institutes of Health (NIH) and the National Institute of Occupational Safety and Health (NIOSH) of the Centers for Disease Control and Prevention (CDC), including R01 0H003954, R01 NR007677, and R01 HL086934 to Charmane I. Eastman, PhD, and R01 HL083971 to Helen J. Burgess, PhD. The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH, NIOSH, or CDC. The NIH, NIOSH, and CDC had no role in study design, data collection and analysis, interpretation of the data, and in the preparation, review, or approval of manuscript.

We thank the following people for their assistance with data collection: Daniel Alderson, Elizabeth Beam, Jillian Canton, Young Cho, Erin Cullnan, Meredith Durkin, Corinne Eckstein, Valerie Ellois, Clifford Gazda, Cynthia Hiltz, Heather Holly, Hyungsoo Kim, Clara Lee, Kathryn Lenz, Vanessa Meyer, Jacqueline Munoz, Courtney Pearson, Meredith Rathert, Victoria Revell, Mark Smith, Christina Suh, Jonathan Swisher, Christine Tseng, and Nicole Woodrick. We thank Stephanie Crowley, PhD, for blindly rating the thresholds and for her comments on the manuscript, and Louis Fogg, PhD, for his statistical advice.

Declaration of Interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.