The hypnogram (on top) ist the traditional expression of sleep depth. Originally it is based on a set of visual sleep stage scoring rules which classify the sleep to wakefulness, rapid eye movement (REM) sleep, non-REM sleep stages S1 (lightest sleep) to S4 (deepest sleep) and movement time (MT).
The other two curves are sleep depth measures based on the computer analysis of
the electrical activity of the brain (EEG). These curves plot the mean frequency of the EEG
using two different calculation paramenter settings. Computers are able to calculate these
in seconds whereas it takes 2-3 hours to produce the hypnogram carefully visually of the same
More information about this topic can be found in our article
"Systematic performance evaluation of a continuous-scale sleep depth
measure" by Antti Saastamoinen, Eero Huupponen, Alpo Värri, Joel
Hasan and Sari-Leena Himanen in Medical Engineering & Physics, Vol. 29, (2007) pp. 1119–1131.