It is even more weird than that. From my and some friend's experience: if you sleep more then 9 hours straight, instead of usual 6-8 (this happens on weekends usually), I feel really terrible in the morning - not being able to get out of bed, but in the same time not being able to sleep. Finally when I get out of bed - I feel really trashed for next several hours.
So it is not just less than 8 hours makes things worse, same applies to more then 8 hours. So it is either I have to sleep 6-8 hours only, or I have adapted to this schedule so well, that anything else makes me feel terrible.
On the other hand, I have handful examples of me traveling with few flight changes, timezones changes, more than 30 hours in flights/airports, and after getting to my destination I spent full 16 hours day working or meeting with friends just fine. May be a little dizzy. And after consequent 8 hours sleep night - feel great next day.
This suggest, that it (8 hours sleep requirement) is more than just a something "hardwired" in to human's body...
Often when you wake up after sleeping longer than usual (say more than 9 hours) you feel tired because of dehydration. Your body loses a fair bit of water while you sleep, so that might be reason you feel bad after a lie-in.
I have anecdote to confirm this. I stay well hydrated, and drink more water than most people I know, will generally sip on a glass by my bed, and have a glass of water by my bed at night. I have also been able to sleep over 12 hours (generally waking up at least once to use the restroom and water) Yet I feel fantastic when I do such!
This is why, I find that getting up at the same time on the weekend as I do during the work week actually helps keep my sleep patterns somewhat regular. So, although the idea of an occasional Saturday morning sleep-in sounds good, in practic, I'm better off getting up at my usual 4:45am every day and supplementing with a short nap after lunch if necessary.
So it is not just less than 8 hours makes things worse, same applies to more then 8 hours. So it is either I have to sleep 6-8 hours only, or I have adapted to this schedule so well, that anything else makes me feel terrible.
On the other hand, I have handful examples of me traveling with few flight changes, timezones changes, more than 30 hours in flights/airports, and after getting to my destination I spent full 16 hours day working or meeting with friends just fine. May be a little dizzy. And after consequent 8 hours sleep night - feel great next day.
This suggest, that it (8 hours sleep requirement) is more than just a something "hardwired" in to human's body...
Yes, sleep is weird.