In Reply Drs Abbott and Zee comment on the possibility of lower urinary melatonin levels serving as a marker for underlying circadian misalignment related to cumulative shift work. We have also contemplated the possibility that melatonin levels are simply a reporter of circadian alignment rather than a marker for abnormal glucose metabolism.
Melatonin levels among individuals in the NHS have been shown to be inversely associated with the number of night shifts performed in the 2 weeks preceding the measurement,1 whereas no association was found between melatonin levels and the total number of years of previous shift work for individuals in the NHS.2 This suggests that shift work likely does not have a sustained effect on melatonin secretion.
McMullan CJ, Schernhammer ES, Forman JP. Melatonin Level and Risk for Type 2 Diabetes—In
Reply. JAMA. 2013;310(5):537. doi:10.1001/jama.2013.7655