You want to shoot for 20 minutes or 90 minutes, nothing in between!
20 minute naps (set a timer between 15 or 30 minutes) are short enough that sleep inertia doesn't affect you much. Sleep inertia is what makes you feel like shit after a long nap. You will not feel like your are sleeping in that 20 minutes. It has big "resting my eyes energy." But just 20 minutes has huge benefits and will allow you to feel alert the next few hours. No grogginess.
90 minute naps are the longest naps you want to take. A full sleep cycle (going from light to deep sleep and back) takes about an hour and a half. 90 minutes guarantees you wake at a light stage and you'll be able to shake off that sleep inertia quick.
Both of these options are not likely to spoil your normal night's sleep either. Source for this info at the link.
Bonus nap: The Caffeine Nap 😈
Drink a stimulant (coffee, red bull) and immediately go down for a 20 minute nap. By the time you wake up, refreshed, the caffeine will have metabolized and gives you an additional boost of alertness. This is how I survived my last few semesters of college.