© American Journal of Hypertension, Ltd 2019. All rights reserved. BACKGROUND Ethnic differences in nighttime blood pressure (BP) have long been documented with African Americans (AAs) having higher BP than European Americans (EAs). At present, lower nighttime melatonin, a key regulator of circadian rhythms, has been associated with higher nighttime BP levels in EAs. This study sought to test the hypothesis that AAs have lower nighttime melatonin secretion compared with EAs. We also determined if this ethnic difference in melatonin could partially explain the ethnic difference in nighttime BP. METHODS A total of 150 young adults (71 AA; 46% females; mean age: 27.7 years) enrolled in the Georgia Stress and Heart study provided an overnight urine sample for the measurement of 6-sulfatoxymelatonin, a major metabolite of melatonin. Urine melatonin excretion (UME) was calculated as the ratio between 6-sulfatoxymelatonin concentration and creatinine concentration. Twenty-four-hour ambulatory BP was assessed and nighttime systolic BP (SBP) was used as a major index of BP regulation. RESULTS After adjustment of age, sex, body mass index, and smoking, AAs had significantly lower UME (P = 0.002) and higher nighttime SBP than EAs (P = 0.036). Lower UME was significantly associated with higher nighttime SBP and this relationship did not depend on ethnicity. The ethnicity difference in nighttime SBP was significantly attenuated after adding UME into the model (P = 0.163). CONCLUSION This study is the first to document the ethnic difference in nighttime melatonin excretion, demonstrating that AAs have lower melatonin secretion compared with EAs. Furthermore, the ethnic difference in nighttime melatonin can partially account for the established ethnic difference in nighttime SBP.