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.