I´ve heard a lot of people talking about having an LD in a nap.
my question is, are you always in REM if you take a short nap during the daytime, or do you have to nap for a certain amount of time (90-120 minutes for example), or is it random if you are going to have a REM period.
I think it depends on what phase of sleep you need. From what I understand, the brain prioritizes the type of sleep that it needs most, first. For most people in the Western world, we skimp out on the last hour or two of sleep, which is the most REM heavy. This means, if you sleep 6-7 hours and get up, when you take a nap you're most likely to rebound on that REM sleep you missed. Missing that last hour or two of sleep is actually devastating for our sleep and, as a result, our health. When daylight savings time shortens our sleep by an hour, there's an uptick in heart attacks on that day, while the opposite is true when we gain the extra hour of sleep. Regardless, if for whatever reason you happened to have less deep NREM sleep the night before, your nap would put you into that area of sleep that was lacking first. Though, it's important to remember that dreams do not exclusively happen in REM sleep, they just happen there more reliably. Dreams have been recorded in NREM sleep in the lab before, so it's possible you could have a dream during your NREM nap, but it may not be as robust or ripe for lucidity as normal REM dreams are.