Here is a quick argument that sleep is bad in some respects. Sleep either involves unconsciousness or non-veridical experiences. Unconsciousness is bad, since it is the lack of consciousness, which is a good, and a good due to our nature as rational. Non-veridical experiences are clearly a bad. So, sleep always is bad in some respects.
Whether this argument succeeds or not (I think it doesn't; from our nature as rational beings it does not follow that it is our nature to always exercise rationality), it does raise a question about the value of sleep. Clearly, sleep is instrumentally good. Is it good non-instrumentally, though? And will we sleep after the resurrection of the body? As one of our grad students pointed out, Scripture considers sleep analogical to death. There are also positive portrayals of wakefulness. So when death is no more, will there be sleep? Aquinas thinks not.