system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that when a person takes a nap in the afternoon/evening they wake up feeling incredibly gross and feel more tired after?

This happens to my girlfriend and I often. We'll take a nap then 15-30 min later when we wake up we feel more tired and kinda gross. Sometimes I even feel that my senses are heightened or some rush is going through my body. I've read that taking a nap later in the day is not good because it'll keep you up later in the night as you try to sleep. But never really read anything about the feelings I get after taking these dreadful naps.
gpt
Sleep inertia. If you are going to nap less than 90 minutes (a full REM cycle), *only* do it for 15 or less. You will start entering deeper sleep and be groggy afterward otherwise. Bonus for a caffeinated power nap!