This is called sleep inertia, and happens when you wake from a deep sleep. (inc.com)
These naps include all the stages of sleep, and by ensuring you complete the full cycle you will avoid the effects of sleep inertia. (treehugger.com)
A nap that is long enough to include a full sleep cycle, at least 90 minutes, will limit sleep inertia by allowing you to wake from REM sleep. (inc.com)