You probably feel more energetic with the shorter periods of sleep because you are waking up around the end of a cycle. If you wake up in the middle of a sleep cycle you will feel groggy. There are some products on the market that capitalize on this by waking you up when you are most restless (basically equates to lightest sleep). One of the products was recently featured in popsci. I'm sure if you want more information you can find it online.