He was a professor of psychiatry and a pioneer in sleep research. He ran early studies of rapid-eye movement sleep (REM), which he identified as the stage during which most dreams occur. He described the phases of the sleep cycle, using electroencephalograms to monitor brain waves. He established the health risks of sleep deprivation and sleep apnea. He wrote one of the first college textbooks on sleep, and founded the Stanford Sleep Disorders Clinic, considered the first sleep lab in the nation.