Participants are admitted to the hospital for two 12 hour overnight stays, during each of which they provide a 24-hour urine collection and have a small blood sample drawn every 20 minutes for 12 hours (from 8:00 p.m. to 8:00 a.m.) through a catheter that remains in place in a vein. Blood pressure and pulse are monitored during this time. After the catheter is removed, subjects complete questionnaires about their pain, mood and activity.