I don’t think 8 hours is a myth, but more like that was what was observed in the 60s when the Stanford Sleep Research Centre by William Dement ran their first experiments on keeping people in white rooms without stimulation until their bodies found a natural rhythm.
It would be interesting to know if those experiments have been replicated and if there were any deviations!
What's interesting is my daughter has similar symptoms as you and has been diagnosed with Delayed Sleep Phase Disorder (DSPD). She's recently been prescribed a dose of melatonin which she takes an hour before the time she wants to go to sleep.
That appears to have helped somewhat.
But for me, 5mg does tend to knock me out and I can barely stay awake after an hour of taking it.
https://en.m.wikipedia.org/wiki/Delayed_sleep_phase_disorder
That one sleeps 4-5h per night also sounds like one light/deep/REM sleep cycle, of which there would typically be two in an idealised 8h-ish "night".
There are hypotheses out there that a single 8h block of sleep is a myth - possibly socially induced by industrialisation and 3x8 shifts - and that waking up in the middle of these 8h is a normal thing.
"waking up" here may mean anything from near-consciousness sleep to actually waking up and possibly do shit for an hour then going back to bed; or even for some, splitting sleep in two 4-hour blocks spread around the 24h day.
In parent's post it may be that one 4h block is done, then social pressure (work/child schedules) pushes one to continue with this one-block schedule.
It may even look appealing as triggering sleep through exhaustion superficially appears to help with sleep in the short term. Long term it gets exhausting but this perceptually becomes the new normal, especially when the symptoms of this kind of sleep deprivation aren't that obviously tied to sleep habits, the naively expected ones being masked by external pressure (schedule), habit bias (normalisation of deviance through repetition), exogenic (alcohol, caffeine, nicotine) or endogenic (adrenalin, cortisol, dopamine).
The main problem is then that by the time symptoms are impacting the situation is deeply anchored; worse, because the root cause is non-obvious it is often misdiagnosed.
Don't ask me how I know.
Also, not a physician, just saying: take care.