Unless you have a CRT monitor that stays on all the time from before the Energy Star days (long ago) -- the computer won't use too much energy when it's asleep.
The power bricks for your cell phones, VCRs, cable tuner boxes, will likely be using a whole lot more energy than a sleeping PowerMac MDD.
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Basically if you wake up in the morning, and the box is cold -- it's not spinning the meter -- unlike some other things which are warm even when turned off.