Fig. 1. Schematic diagram of the experimental protocol. Groups 1 and 2 received intravenous morphine at times 09:00 h (group 1) or 18:00 h (group 2). Groups 3 and 4 received placebo at times 09:00 h (group 3) or 18:00 h (group 4). The  R s indicate the times of respiratory testing. The dark period is the sleep period. In groups 2 and 4, blood sampling continued during sleep, but no other data were acquired.