<off topic, sorry>
About changing your sleeping hours - I've just been doing that, following Steve's suggestion of always getting up at the same time, and going to bed when you're tired. It works, but you can expect to spend some time half-asleep while your body adjusts.
As for controlling the length of naps during the day, I found that
a) like you, if I just 'intended' to wake up in 20 minutes I'd be off for an hour or more
b) if I set a timer, I'm too anxious about when it'll go off to get to sleep properly at all
c) if I listen to a brainwave entrainment session of the right length through headphones, I wake up naturally and gently when it ends (or when the track changes).
It worked for me - perhaps it'll work for you, too.
</off topic, sorry>
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