If you wake up in the middle of a sleep cycle, you will feel really groggy. There's even some evidence that cognitive activity between the hours of 4 and 6 a.m. is very productive. It's easier for me to wake up with 6 hours of sleep than it is for me towake up with 7 to 8 hours of sleep. And it's because you have to wake up during the right time of your sleep cycle.