Is it possible to have an LD during the day, maybe from a 2-3 hour nap in the afternoon or so? Or does it only happen during REM cycles at night?
Yeah, it’s very possible. During day naps you enter more quickly in REM stage.
Some people would even say it’s easier to LD during daytime naps because of how much quicker you go into REM
I second what Ysim said. Personally I never had an LD during naps, but I take naps very very rarely so that doesn’t say all that much.
On the other hand I can confirm that naps can easily yield dreams with the qualities of the last REM cycle of a whole night’s sleep. Pretty awesome stuff may happen.
If you are contemplating to explicitly add naps to your sleeping schedule to increase your chance of LDs, I am not sure if I agree to this. I think it is worth trying out biphasic sleep (doing a nap throughout tthe day next to the main sleep at night) to see if this is overall a healthy sleeping pattern for you. But if you find that monophasic sleep feels better to your body, then I recommend doing that. Don’t put lucid dreaming before your physical health.
If on the other hand you are already doing naps anyway and are wondering if it is worth attempting a technique during naps, then go all out I’d say