I sleep for 11-14 hours daily; alarms do not work (I turn them off without realizing); have extreme sleep inertia—can wake and fall asleep many times before finally getting up; am extremely tired all the time, and have no motivation to do anything.

I also have somewhat very lucid dreams every morning, that I can clearly remember for quite some time after waking up.

Approximately, every 3-4 months these symptoms disappear for a ~few weeks: I sleep again for 7-8 hours, wake up without alarm, full of motivation, basically blooming and life-affirming. Then the cycle repeats again.

Scientifically speaking, what could it be?