Eight healthy human subjects were enrolled in a 6-day simulated shift work protocol. Blood samples were collected during the two 24-hour measurement periods. Blood samples were collected every 4 hours during both measurement periods. Subjects entered the lab on Day 1. At the start of Day 2, the first 24-hour measurement period was started. Subjects slept according to their habitual sleep/wake schedule, followed by a 16-hour constant posture procedure. On days 3-6, the sleep period was delayed by 10 hours. Following the third night on this schedule, subjects underwent another 24-hour measurement period. During both measurement periods, 7 blood samples were collected and PBMCs were isolated. mRNA was extracted, labelled, and hybridized to microarrays.