i thought you never woke a sleeping baby, until my dd and i were rushed to hospital when she was 5 days old because she was on the way to dehydration. i obviously wasn't making that good a job of feeding her when she was awake, and she just slept all night and i let her.
i was told in hospital 'every 3 hours', which i stuck to religeously (sp) for a few days, till the visiting mw said that i could leave it 4-5 hours at night. i took this as gospel and left it 4.5 hours (that fitted in with feeding at virtually the same time every day too - 6, 9, 12, 15, 18, 21, 01.30, 6 again. i even set an alarm to wake myself until she was 10 weeks old!!!
even when i stopped setting the alarm, she only had to flap about a bit (ie was not really awake) for me to awaken and feed her, but i was a little neurotic by that stage.
my mantra has always been 'when in doubt, feed them'. you can't overfeed a breastfed baby, and even if you think she only just ate, she might be hungry again...
what i am trying to say is, if you are sure you've fed your lo every 3 hours or so during the day and you're happy with what they've had (i think you know whether they've had enough or not) then you can risk leaving it a bit longer at night.
don't give up, you're doing really well, and it's better to be overcautious than like i was to begin with.
good luck!