Quickly going into REM when asleep is a sign of either narcolepsy
But only people with excessive daytime sleepiness are tested for quickly going into REM, so the fact that they do doesn't tell so much. Anecdotally, I find that people with narcolepsy went quickly into REM before they developed the excessive daytime sleepiness. They seem to function quite well, until they develop full-blown narcolepsy. So I don't think it is reasonable to associate quick REM with narcolepsy. Sleep deprivation is another matter.
Yes, what I wrote was too restrictive. Sleep-onset REM periods (SOREMPs) are definitely a part of normal experience. If they occur regularly in an individual, that's a symptom of narcolepsy (or sleep deprivation; the two are similar). There could be possibilities that I am not aware of, as well. I know people who have claimed to have SOREMPs sometimes who don't have narcolepsy and were not sleep deprived. I was not arguing that SOREMPs are necessarily bad, just that what is known about them is not good, which set off a red flag for me.
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