It seems that the optimal length for a nap is equal to the initial period when you first fall asleep and are in light sleep. Once you transition to SWS Deep Sleep, you are likely to experience a sleep hangover when awakened, so it’s preferable to terminate your nap before entering deep sleep.
Surely there should be a way you can start a ‘nap’ activity with a setting that triggers the haptic alarm as soon as Whoop detects you are entering deep sleep. This would be so much more effective than just setting an alarm for 20-30 minutes after you try to nap, because if you’re like me you may not fall right asleep right away. This would be the perfect way to ‘optimize’ a nap activity. While I never use the Whoop alarm to wake me up after a night’s sleep, I can definitely see myself using this feature for naps.