How Did a Much Later Bedtime Leave Deep Sleep Short?

On Huberman Lab, a night that begins at 3 a.m. rather than at 10:30 p.m. does not follow a fixed program that always opens with the first couple of hours of deep sleep. Sleep mostly moves into rapid eye movement phases and stays significantly deficient in deep sleep.

Some deep sleep still arrives at the start, because a longer time awake creates significantly greater pressure for deep sleep. The same split is an experimental technique. First-half-of-the-night deprivation, then sleep in the second half, leaves a person mostly deep-sleep deprived while they still get mostly all of their rapid eye movement sleep. Restricting sleep to the first four hours yields mostly deep non-rapid eye movement sleep and almost no rapid eye movement sleep.