I don’t know, but I have had the same problem. It could be that when someone sleeps for 12 hours that the person does not go into the deep sleep stage as often in the night, and can wake up unrefreshed.
If you’re like most people, dehydration will make you feel tired regardless of the number of hours you sleep. Typically, you’ll spend all day nodding off and will feel just as tired when you wake up as you did before you slept. This doesn’t really answer your question, but it might be what you’re noticing.
You also have to ask the question - why did I oversleep? Don’t always ascribe the symptom (oversleep) to the cause. Both the oversleep and the grumpy tiredness may be a result of the same causative factor. Simple (and potentially nasty) things like sleep apnoea might be an issue, or you could simply be ill.
Waking up in your REM sleep cycle can make you feel tired. Know your sleep cycles and time yourself to wake up in a non-REM stage, and you should be fresh and energetic (assuming you are getting enough sleep). I had a similar thread here.
There’s got to me more to it than that. Very frequently, I will wake up early, feeling awake and refreshed as I go to the bathroom and ponder what to do next. If I decide to stay up, it is great, and I get a lot accomplished - a great start to the day. But when I choose to go back to bed for whatever reason, I’ll get up a few hours later with that grumpy feeling. It seems to me that I really did not NEED the extra sleep, because I could have stayed up if I chose to. The grumpiness seems to result from waking in the wrong part of the cycle, despite that I did get enough sleep.
I might set my alarm for 7:30 and wake at 7 feeling OK to get up, but insisting on getting my last half an hour of sleep. When the alarm goes off at 7:30, I’ll feel dead and want hours more.
I haven’t been able to achieve this much, but I have had a few periods where I’ve established enough of a routine that I tend to wake naturally around the time my alarm goes off anyway. That’s vastly preferable, in my experience. I think sometimes you just need some discipline in when you go to bed and when you get up for a few weeks, even if you’re forcing it, until your body adjusts.
I discovered years ago that if I slept more the 8 hours I would wake up with a bad headache that would persist all day despite taking pain killers. I can wake up after 6-7 hours sleep and feel good all day.
This. And a little experimentation. If you are getting healthy sleep, your brain will awaken naturally when it is good to go. If you insist on additional sleep just to obey the clock, you might hit a dreamy REM cycle and wake up feeling like a log.
Based on personal experience, I cannot adequately emphasize how important this is.