The video above, from ASAP Science (worth subscribing to on YouTube if you dig videos like this) explains how sleep cycles work, and how power naps—or those naps that do the most to boost cognitive function during the day—take advantage of the first two phases of your sleep cycle: stage one, where you're probably "dozing," or feel relaxed but if someone woke you you probably wouldn't even notice you'd been asleep, and stage two, where your brain starts to consolidate memories, organize its biological bookshelves, and shuts the brain off from external, non-dangerous stimuli. If you're the type who says "It takes me 10 minutes just to fall asleep," that 10 minutes is probably leading you into stage one—after that, you're in stage two. The trouble comes in stage three, or the part where we're sleeping deeply, and waking is difficult. That's when you start to feel groggy, and hate the idea of getting up. If you hate mornings, you're probably waking up during this phase. So the key to getting all of the benefits of naps without the drawbacks is to sleep only for about a half-hour, or the time it takes your brain to go through the first two stages, but not enter the third.
