Hello weajon! REM sleep occurs during the fifth stage of sleep. It takes several hours for the REM cycle to really start up. You will then start hitting REM every 90 minutes or so throughout the night. As the night progresses, the REM period lasts longer and longer. Once you wake in the morning, your mind has gotten used to the sleep cycle throughout the night. It can easily fire off right back into the pattern if you were to attempt a nap. As the day carries on, your mind/brain gets more and more used to being awake, until the REM sleep cyle that your brain was keeping in its "back pocket" is so far removed that it becomes very hard to enter directly. The longer you wait during the day to perform your nap, the harder it gets. Hope that made sense. If you have anymore questions, don't hesitate to ask.