This data is pretty dated. Your brain compensates. There's something called REM rebound to where if you don't get enough sleep, your brain will offload as much REM as possible to the end of your sleep, filling in the missing and crucial sleep cycle.
Is your brain a separate entity? You literally decide when to wake to with alarm. Or if you don’t set a time, it wakes you after x hours, after it’s backloaded all the REM.
If sleep deprivation gets too bad, it’ll completely knock you out to get as much REM in as it feels you need to function, alarm or no alarm.
Weird polyphasic sleep schedules take weeks to start working, so probably from patterns. Although if what op comment is true something else is happening too