PCA (Patient Controlled Analgesia) is a delivery system for pain medication that makes effective and flexible pain treatments possible by allowing patients to adjust the dosage of analgesics themselves. Unlike previous research on patient controlled analgesia, this study explores patient demand behavior over time. We applied clustering methods to disclose demand patterns among patients over the first 24h of analgesic medication after surgery. We first identified three demand patterns from patient controlled analgesia request log files. We then considered demographic, biomedical, and surgery-related data to evaluate the influence of demand pattern on analgesic requirements. We recovered several associations that concurred with previous findings, and discovered several new correlations.