I think even more interesting is when I wake up before my alarm and actually get up/out of bed I feel like I had proper sleep. However, if I decide to go back to sleep for the short period of time before my alarm, I usually wake up by the alarm and feel like I got terrible sleep.
Yes! Basically, every time I wake up because of alarm I feel disgusting.
But the most bizarre thing for me is that I pretty much always get up before the alarm (and hence feeling perfectly fine) if I drastically changed my regimen recently (which normally is pretty much non-existent), even if it leaves me with 5 hours of sleep. But once I get comfortable with it, I start sleeping longer, waking up after the alarm (even if I set it purposefully late, making place for, like, 9-10 hours of sleep), waking up tired and hating my life.
This phenomenon is known as “sleep inertia”: if you’re awakened during certain stages of deep sleep, you’ll generally feel foggier and tired for longer after waking up.
That's a lot simpler. The deeper asleep you are, the less pleasant it is to be dragged out of sleep. When you wake up on your own, that effect is basically at zero.
I couldn't find a reliable reference quickly, but I believe humans have a "90 minute sleep cycle." (Search for that if you wan't a bunch of half-baked hints.) If you wake up (and get up) during the correct phase of the cycle, then you'll feel good. Otherwise you won't.
