Sleep microstructure
Disclosure: This is human writing based on LLM breakdowns and data analyses.
This is a hypnogram of one sleep with both the macrostructure and microstructure included. Zoom in.
Microstructure is dense and random. An average night has 411 Phase A pulses, so many I had to switch to vector graphics to show them. It also influences macrostructure - an A1 pulse helps soothe you into deep sleep, and a strong A3 pulse might spike you to N1 sleep - but hypnograms miss these <30 second long fluctuations.
Why does the average sleep have so many and so successful A3 pulses? Or in other terms, why does the average sleep have so much N1? Turns out the brainstem is to blame - whenever it has any reason for an arousal (eg awkward posture), it sends norepinephrine up to the brain and down to the body, spiking heart rate and resolving the problem.
Also turns out you have a literal sleep drive, in terms of the ratio between A1 and A3 pulses. It starts out strong every night (6.6 A1:A3) and weakens by the end (0.86 A1:A3).