Twenty-Four-Hour and Annual Variation in Severity of Pyrexia: A Retrospective Observational Study of Children Attending a Community Pediatric Emergency Department.

Background The body temperature (BT) circadian rhythm (BTCR) of healthy, full-term neonates becomes clearly discernible by six months of age and is prominent by two to five years of age, with the evening peak-to-morning trough 24-hour variation of ~1°C. Methods We retrospectively reviewed the triage BT data of children aged six months to six years presenting with pyrexia - temporal or oral temperature ≥100.4°F/38°C - to a community pediatric emergency department (PED) between January 1, 2018, and June 30, 2023, to explore evidence of the persistence of the BTCR during fever. Results A prominent sinusoidal-like 24-hour pattern in the hourly mean and median triage BT measurements of 5,953 children was substantiated by Cosinor analyses (always p < 0.0001). Severity of pyrexia, in terms of the hourly mean and median BT values, was greater by ~0.28°F (~0.16°C) and ~0.42°F (~0.23°C), respectively, in children presenting to the PED between 21:00 h and 02:00 h than in those presenting between 07:00 h and 11:00 h. Mean and median BT values also exhibited annual patterning, with the highest values observed in November through January, and the lowest ones in June through August. Conclusion These findings support the hypothesis that the endogenous BTCR persists during fever, with pyrexia more severe in the late evening than in the morning, consistent with the findings from prior studies of hospitalized patients. The detected annual variation in the severity of pyrexia suggests the impact of exogenous phenomena, particularly seasonal differences in the occurrence and severity of infectious diseases.