20 minute power naps are awesome if you can get the timing right. I usually use an alarm, because if I nap much longer than 20 minutes then coming out of deep sleep leaves me feeling super disoriented.
20 minute power naps are a lifesaver for me. If I only got around <= 5-6 hours sleep the night before, getting a solid 20 minutes in in the afternoon is enough to make me feel alert and awake until my regular bedtime. Anything over 25 minutes and I feel groggy and lethargic until the next day. Do not go beyond 20-25 minutes in one nap.