It's probably related to sleep cycles, and maybe the issue is waking up at the "normal" time coincidences with the "right" cycle time to wake up, and when sleeping in you wake up because you're topped off, or have to drink or pee or whatever, which may or may not be at a good cycle time.

Try wearing an extra sweatshirt and setting your alarm so you get about 90 minutes of sleep. Most people have sleep cycles around that long, so you’re less likely to wake up in the middle of a cycle, which can leave you drowsy. The sweatshirt is so that your muscles don’t get cold, which can make them achy.

Oh boy. Big topic.

1. Don't refer to "average sleep" of 7-8 hrs. Plenty of people—like myself and everyone on my paternal side—struggle to exceed 6.5 unless post a physical day. Plenty of other people unfathomably need 9 or more hours. "Average" is such a poor yardstick, especially with so many unique outliers. Don't pursue "ideal" sleep if you naturally need more or less.

2. Sleep cycles are around 90 mins. Your thoughts wander into dream state, you go into deeper states, and return to a dream state as the cycle ends. You dream several times a night but will only be aware of it going to sleep and especially waking up. Yet a few mins after waking up, you will have forgotten the details of the waking dream.

3. If you wake up mid-cyxle, not matter how much sleep you've gotten, you will feel drowsy. The brain activates awake, but everything else is in sleep mode and gets stuck there for a while since it never got its natural chems/triggers it uses to wake up This is why if you.re a 7.5 hr sleeper and missed it, it's better to set the alarm to 6 than 7. Multiples of 90mins as a rough rule to avoid feeling shit next day.

4. Listen to your body. If you are hungry, eat. If you are not, don't. Sleep is the same. If you force sleep, you will not benefit, espially if your brain is confused and fires up after an hour into "sleeping in", making you feel drowsy. 3, 4.5, 6, 7.5, 9.

My advice is trial going to bed earlier. If you wake up earlier, you're all good. When you wake up, that's it. Don't try go back to sleep.

On the other hand, if you go to bed early and wake up normal time, enough of that confirms you need the sleep. Your brain may be ready to wake up at 8am, but your body is not. Give the body an hour early mark and see what happens.

I have no answer, but you're not alone. If I get exactly 7 hours I'm golden. 8, I'm fine. ≥9 or ≤6, I'm exhausted all day.

In case 2 it sounds like you sleep less, or less well. Unfortunately it's hard to monitor. Devices like Fitbit are better than nothing but are not very accurate.

I can't nap, it always makes me more tired. Long or short, it never helps.

7 hours is fine, if you wake up rested then 7 hours is perfect for you, and it's sufficient. I like 7.5, anything over 6 hours and under 10 is normal as far as I know.

And as others are saying, waking up in the wrong part of the cycle, it's disorienting and feels like you got less sleep. This happens to me all too often.

I'm afraid it sounds terminal and you probably only have 5-10 minutes to live.

Now, I am not your doctor and you should contact your family GP to get their opinion.