A large body of data has been gathered on the sleep characteristics of normal subjects. The evolution of each sleep stage within each NREM/REM cycle is presented in detail, showing stage intensities minute by minute. There is a three-phase pattern in each stage intensity diagram: an initial phase of rapid change; a central phase of relative stability; and a terminating phase, again, of rapid change. The details of this pattern change progressively during the night. Throughout all cycles, there is a complementary relationship between the intensities of stage 2 sleep and the other stages that underlines the central role of stage 2 sleep in all stage transitions. Stage intensity diagrams for two groups, one group with and one group without stage 4 sleep, were compared. Subjects without stage 4 sleep tended to have a shorter duration and greater latency of stage 3 sleep. Surprisingly, cycles interrupted by abnormally long periods of continuous wake showed a negative correlation between the intensities of wake and slow wave sleep, and these interruptions did not appear to reset the cycle clock to zero. Sleep stage intensity diagrams may be useful to study the sleep patterns of populations of insomniac and depressive patients, as well as the effect of drugs on sleep.