Pain is a bio-physiological phenomenon characterized by a circadian rhythm. A better understanding of diurnal variability in orthodontic pain perception would not only enhance our knowledge about how orthodontic pain intensity fluctuates over the 24-h day, but it also has a great potential to improve the clinical management of orthodontic pain. Since the administration timing of pharmacological interventions has a direct influence on their effectiveness, a sound knowledge of the timing of peak pain intensity would allow clinicians to better coordinate the administration timing of analgesics. The objective of this study was to explore and quantify the diurnal variation in orthodontic pain over a period of seven days following initial arch wires placement. A multilevel linear spline model was used for secondary data analysis. Data were obtained from an earlier published high quality randomized controlled trial involving 85 participants (42 males and 43 females; mean age 14.1years and SD 2.0). Results showed a significant diurnal variability in pain intensity during the first two days of force application for both sexes. However, females showed significantly greater diurnal variation in the pain than males. Clinical and research implications of observed diurnal variability in orthodontic pain perception are discussed.