90 mins? Most of the afternoon naps I do tend to be short - like 15-25 minutes, and that generally refreshes me enough to feel "restarted" for another few hours. I can get through the rest of the day pretty good on that. "Rest of the day" being doing "thinking work" for another few hours - I'm not trying to get another full day or work (or even socializing) after that.

That's the other kind of nap, where you don't actually sleep. That's fine too. The point is that a lot of people will sleep, but set their alarm to wake them up in the middle of their deep sleep and then be surprised that they are so groggy.

thanks. I guess it's usually felt like 'quick sleep' but that's my layman's view from the easy chair. :)

I’ve never yet been able to find the standard deviation data on the research that established that the average sleep cycle lasts 90 minutes. Presumably there’s as much variation across individuals as everything else about individuals

I know the concept of deep sleep and cycles but how the hell could you plan this? Like ok, I'm sleepy due to lack of sleep, now I have a certain stretch of time with no time constraints - how do you know when you will fall asleep, when will enter deep sleep phase and whe will it end? I have no idea about any of those numbers.

There are mobile apps or fitness watches/bands that apparently can tell your sleep state based on your movement etc. I don't know how accurate they are because my phone is cheap with cheap sensors.

You tell these apps what time you want to wake up at the latest, and they will ring the alarm before that time when you are not in deep sleep.