I read somewhere, several years ago, that for most people, getting up after less than 3 hours of sleep is fairly easy, and getting up after more than 6 hours of sleep is fairly easy, but getting up after more three but less than six hours of sleep is very hard - the model was something along the lines of 'after three hours of sleep, human brains assume that there's time for a full night's worth of sleep rather than just a nap'.

I don't have any references, nor any significant amount of evidence that this is the case (though it seems to work for me), but the idea that if you get more than X amount of sleep you need to get X+Y amount of sleep in order to wake up easily seems like a potentially useful theory at least.

I'll propose an experiment:

Try falling asleep at different times, and recording your difficuly-to-get-up on some arbitrary scale. Record (approximately) how much time asleep you get along with this.

The "recommended" 8 hours may not be optimal for your physiology.

Disclaimer: Not a doctor, nor an expert in sleep, in any way... This is just from anecdotal evidence. (Girlfriend sleeps about 5-6 hours a night, and is functional. Friend can't function without sleeping 9.)

If you find an amount of sleep that is testably better than the alternatives, at least this might help.