The landscape of sleep. The spectrogram above, created by Joseph Mietus, demonstrates the difference between stable and unstable sleep. This nocturnal electrocardiogram of a healthy man, 24, shows normal fluctuations between stable (top range) and unstable (bottom range) sleep throughout the night. The top mountain range nearly disappears in people with uncorrected sleep disorders. Stable and unstable sleep patterns appear to be independent of classic sleep staging for the same person, shown in the upper bar: wake, rapid eye movement (REM), and non-REM stages 1 through 4.