skoorb is correct........there is absolutely no way to work more hours and take home less, unless they tax you at a rate >100%...then they'd be taking everything you earned in OT and dipping more into your base hours.
And he is also correct in the tax bracket breakdown....it's only the money over the $20K, or whatever the breakpoint is, that's gets taxed at a greater rate.
What happens is that the percentages can increase because the extra hours worked suddenly make you look like you're earning $45K a year instead of the $36K.......so the money gets taxed at a higher rate, but since you've worked more hours, there is obviously more money....no matter the higher rate.
Tax rates as of 2000:
HERE.
You'll notice at each bracket, it's the amount above the last bracket that's taxed at the next level...topping out at 39.6% as the highest bracket.