For a study, researchers sought to hypothesize that the concentration of NP would change during the day and that this would affect how accurately they diagnosed acute heart failure. The diagnostic perfection for acute heart failure of BNP (B-type NP), NT-proBNP (N-terminal pro-B-type NP), and MR-proANP (midregional pro-atrial NP) was compared among 1,577 daytime presenters versus 908 evening/nighttime presenters in secondary data analysis of a multicenter diagnostic study enrolling patients who presented with acute dyspnea to the urgent care. Central adjudication of the final diagnosis was performed. Within the context of validation research, the existence of a diurnal rhythm in BNP and NT-proBNP concentrations was investigated through the use of hourly measurements in 44 persons who were considered to be stable. BNP, NT-proBNP, and MR-proANP concentrations were equivalent among daytime and evening/nighttime patients diagnosed with acute heart failure (all P values were nonsignificant). In contrast, patients who presented in the evening or midnight were found to have lower amounts of both BNP (median, 44 [18–110] versus 74 [27–168] ng/L; P<0.01) and NT-proBNP (median, 212 [72–581] versus 297 [102–902] ng/L; P<0.01) than patients who presented during the daytime. This led to a greater diagnostic accuracy among evening/night time presenters, as measured by the area under the curve of BNP and NT-proBNP (0.97 [95% CI, 0.95–0.98] and 0.95 [95% CI, 0.93–0.96] versus 0.94 [95% CI, 0.92–0.95] and 0.91 [95% CI, 0.90–0.93]) among daytime presenters (both P<0.01). In the case of MR-proANP, these changes were not seen at all. It was established that diurnal change of BNP and NT-proBNP occurs, with concentrations being lower in the evening and midnight in 44 stable persons (P<0.01). Presenters seen in the evening or overnight have greater diagnostic accuracy than those seen during the afternoon. BNP and NT-proBNP exhibit a diurnal cycle, whereas MR-proANP does not.