Continuous recordings were made of oxygen consumption (VO2) and EEG, EMG, and EOG during two nights of sleep for 20 men who were between 19 and 63 yr. old. There was a linear decrease in overnight oxygen consumption with age, even though the older men slept less and woke more often than the younger ones. On the first night, VO2 was higher, and sleep stages less like normal than on the second. In the second night data, individual oxygen consumptions averaged over the whole night ranged from 138 to 482 ml/min (61-247 ml/m2timesmin). No relationship appeared between stage of sleep and VO2 level, but there was usually a decline in VO2 in the first hour of sleep, and a slow rise in the 2-3 h before awaking. Periodic breathing was observed in the men older than 45 yr.