 There's two things with napping. Firstly, if you have problem sleeping, you should never nap. Because you build up sleep pressure during the day and it ruins that. But naps are good for, so when you have your normal circadian dips that you get in the early afternoon, that's why you feel sleepy after lunch. Then, if you nap during that time, then it boosts your alertness and your concentration. And you only nap for 20 minutes to stop you from going into the deepest stages of sleep. So 25 minutes you can sort of get away with stage one and a bit of stage two. But if you are trying to wake up from stage two and stage three sleep, so the deeper portions then you get this thing called sleep inertia. And you feel terrible and groggy and you lose the benefits of, so rather than just a quick refresh and then you're ready to go in your concentration and have things back to sky high, you wake up feeling terrible and groggy and awful.