"During the first sleep cycle, our body enters deep sleep for the longest period of time, it's at this point that our body temperature begins to drop really low, our respiration, heart rate and blood pressure decreases.

If you've ever been woken up out of “deep sleep”, you know that it’s almost impossible to get up.

It takes about 45 minutes to enter the first deep sleep phase. If you limit your nap to 45 minutes, you will sleep mainly in Stage 2 sleep. Stage 2 sleep also plays a major role in restoring physical energy." -- Powerful Sleep – Secrets of the Inner Sleep Clock