You get tired depending on what you do as well, not to mention how you're feeling, what you've eaten, or rather what you haven't, any stress or depression, medical issues such as diabetes, whether you're hot or cold, and last but not least, whether you're bored or not.
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Personally I find that if I go to sleep now (as I will be in ~30 minutes) I can get up for work at say, 7.30 fine. However on a sunday night/monday night (days off) if I stayed up till 3am I'd sleep much 'longer' into the next day. For example :
Sleep at 11.30 - Wake up at 7.30
Sleep at 03.00 - Wake up at 14:30
or something like that (??)