Originally Posted by David Hausladen
micth: Yeah, in most circumstances any nap under 45 minutes will not get into deep Stage 3 or 4 sleep, so while it can be refreshing in the short term, it's not enough to affect the amount of deep sleep you need at night; nor is it likely to "trap" you in a deep sleep that you feel tired waking up from. (The exception is that polyphasic sleepers, from what I've read, reach deep sleep much more quickly and visit different stages in different naps so to get the right amount of each. This explains why polyphasics can get by with just short naps.)
I seriously reduced my sleep time in the night by taking an evening nap. What additionally comes into play here is that you condition your body to get naturally tired in the evening (for me around 7 pm) and you actually fall asleep. If you are not acustomed to napping at this time you might end up trying to fall asleep for 30 minutes but not get any. I think that even a 30 minute nap affects my total sleep time.