We’re actually preparing to wake up around an hour and a half before we actually do wake up. The body starts preparing to wake up. Jan Born did some work in this very area a few years ago.
Essentially if you tell your brain that it’s important for you to wake up at 7am - basically by setting the alarm for that time - your brain recognises that you’re waking up at 7am and it starts preparing for that. It may not like the sound of the alarm so it will enable us to wake up naturally before that time.
https://www.youtube.com/embed/G4c1TIis0Ps?wmode=opaque
And this is where the advice that we give to have a regular wake up time is key. If you do that the body gets into that routine. It knows that whatever time you go to bed that you need to get up at the same time so you can fill that time most effectively with sleep and it will also wake you up at the requisite time.
https://www.youtube.com/embed/iNpXCzaWW1s?wmode=opaque
If you set your alarm even earlier than normal - say, you’ve got to get up for a flight at 4am - the reason you’ll often have a fitful night’s sleep is that you’re scared your alarm won’t go off. Again, by setting the alarm at 4am you’ve actually told your brain that it’s really, really important that you wake up at that time. Your brain then becomes anxious that you might not. ‘Did I set the alarm? Is it going to be loud enough? Blah, blah, blah’. So you either deal with the anxiety or you set two alarms. That way you won’t need the two alarms but you’ve reassured your brain that you’ve done enough to wake up for that important appointment.
"Your brain processes this information and your body knows when you have to wake up"
The act of setting the alarm sets off this chain of events. Your brain processes this information and your body knows when you have to wake up. People say ‘Oh, isn’t that really clever.’ But it’s not really. Your alarm clock knows to go off. And that’s just a simple machine. Your brain is vastly more complex than a simple machine and has a clock within it that is just as accurate. So it should be no surprise that your brain knows when it’s five to seven.
"If you’re going to have a regular wake up time it needs to be seven days a week"
This is why when we wake up at 7am every weekday but not at the weekends we get those Monday morning blues. You have fairly regular habits during the week, then it goes to pot at the weekends. Your brain starts to think you’re on holiday. It was termed social jetlag. If you’re going to have a regular wake up time it needs to be seven days a week and if you do that you will wake up as refreshed and with as good a night’s sleep as possible.
https://www.youtube.com/embed/MJK-dMlATmM?wmode=opaque
Because your body wants to wake you up in a particular stage of sleep, somewhere within 15 minutes of your allotted alarm time is a good natural prediction by the body to wake up. It wants to wake you up in REM sleep - rapid eye movement - that’s when it’s natural for the body to wake up.