Sleep is really important. You need to rest the physiology to be able to work weel and meditate well. When I don't get enough sleep, my meditations are duller. You may even dip into sleep at the beginning of your meditation, because you're settling down. But if you're well rested, you'll have a clearer deeper experience.
Source: Catching the Big Fish: Meditation, Consciousness, and Creativity, Pages: 161
Contributed by: ~C4Chaos