I have the same issue after an acute long term sleep deprivation (slept 9 hours in 3 days with nearly no REM) 8 months ago. It turns out it damaged my thalamus (which is responsible for the wake-sleep alternance). Nothing can be done (at least currently) which is quite depressing, so I cope by taking long acting benzos which allow me to sleep 5-6 hours. From my research it also entice decreased ability to create « sleep spindle », if your sleep is also not qualitative and you’re not stressed, this may be an indication this is the same cause and you may want to see a neurologist.