Hello & Welcome, Efjaristos!

Unrefreshing sleep can grow into a monster. Sure, it may be related to Depression or Obstructive or Central Sleep Apnea, but it may rise from other sources, too.

(Even if the physical structure of your throat rules out Obstructive Sleep Apnea, it wouldn't necessarily rule out Central Sleep Apnea, which involves neurology.)

If possible, get a nighttime Sleep Study followed by a daytime Multiple Sleep Latency Test (MSLT). These tests will give you insights into your "sleep architecture" - the nature of the stages of sleep you pass through during the night and even during the day. It is possible your deeper sleep stages are deteriorating, which happens with aging but can happen more rapidly in some people.

In my own case, circadian rhythm problems are intertwined with depression.

I wish you well. Please make yourself at home here.