Sometimes, when I close the lid on my macbook, and reopen a few hours later, the following happens: While the macbook (unibody) is closed, the breathing light (the little white on the front) is lit solid (i.e. not coming on and off as during normal sleep). When I open the lid, the light goes out, and the macbook is completely unresponsive. The only way to get it up and running again is to hold the power button down until it shuts down, then restart. Why does this happen?