Reading a night as a staircase of stages
A hypnogram plots sleep stage against time, turning a whole night into a single line that steps between wakefulness, the three NREM stages (N1, N2, N3), and REM. Sleep usually descends quickly into the lighter stages and then into N3 deep slow-wave sleep, before climbing back toward lighter sleep and the first REM period.
These cycles repeat roughly every 90 minutes or so, though the length is variable. A typical night contains four to six cycles rather than a fixed number.
Why the night is front-loaded with deep sleep
The balance of stages shifts as the night goes on. Deep N3 sleep dominates the early cycles and fades later, while REM periods are short at first and lengthen toward morning. This is why the first half of a night is richer in slow-wave sleep and the last hours are richer in REM and dreaming.
Individual hypnograms vary with age, sleep debt, alcohol, and many other factors. This is general educational information about sleep architecture, not a diagnostic tool.