IB is really hard, a girl I know went to a different sixth form to do it and she's ridiculously smart and struggles with ti. A-Levels are a bit more straightforward and easier to understand. If you really want to push yourself- go for the IB, but if you feel you won't be able to work that hard go for A-Levels. I do A-Levels, I quite like them. ^^
IB is harder but it looks better . . . I think. I did A Levels and enjoyed them a lot and you get the chance to specialise which you can't do with the IB.
Ask your teachers what they think would be better suited to your learning style. And good luck! And good on you for thinking it through, it's important stuff!
Sleep isn't important anyway (Y)