It depends on your worldview. I infer that you perceive "aliens and daemons and spirits" as less simple because the possibility that these things exist does not seen plausible to you. However, consider a hypothetical scenario: Imagine that you did believe that such things were possible.
With that belief, which would be the more simple scenario:
A) Due to poorly understood, but purely physical and/or biochemical causes...people throughout history have reported nighttime visitations by creatures. There are distinct trends and similarities amongst the descriptions of these creatures as well as similarities in their behavior. But it's a complete coincidence, and people are really just hallucinating these creatures rather than actually seeing real ones.
B) These creatures prey upon humans during a time when they're vulnerable and sometimes people notice.
Which is simpler?
It's not about things being "possible". I believe that aliens are possible. That does not mean that aliens are a simpler explanation for this, for example, than normal physiological/psychological processes. That is damn silly.
You have to understand how occam's razor actually
works. If aliens are responsible, for instance, that
raises a hell of a lot of questions. Since when are aliens doing this? Why? Where are they from? It introduces
more undefined variables and raises more new questions than a simple biological explanation, whether you think aliens are a plausible thing to exist or not. For example, I believe that it's possible for the government to shoot people and cover it up if they were to try hard enough, but that doesn't mean it's reasonable to assume that they're responsible for some random murder when there's no particular evidence of that sort of connection.
It's also not implausible at all, nor should it seem so, that people across different cultures and eras would experience similar things, because people are made out of the same stuff and have similar biological/neurological processes. Hell, fundamentally, we're mostly the same throughout. We experience many of the same things every day, in all cultures, common or not, whether it's hunger, needing to go to the bathroom, deja vu, having a word on the tip of our tongue we just can't get at, having dreams about certain common themes, or any number of little biological or psychological things that are common across the globe. People sleep, dream while they sleep, sometimes hallucinate while they're falling asleep/waking up, and can't move while they're dreaming. It's extremely plausible that something like sleep paralysis is just one of those things common to how the human brain works every once in a while, just based on those simple facts. More importantly, it
doesn't introduce new undefined variables or raise more questions. That's why it's the "simpler explanation". The real, material explanation of sleep paralysis fits perfectly well with what we understand about the world, is consistent with it, and does not really raise further questions that need to be answered. The other interpretation of this: It requires fewer assumptions of what those unknown variables
are. If you figure it's aliens, you're assuming that aliens are not only possible, but real, that they can access Earth, have some sort of reason and capability to do these things to people, and succeed without being detected beyond the vague feelings interpreted by people... and probably some other things necessary for this to be true, that we don't know. Those are huge leaps to take when determining whether or not a given explanation is plausible. The physical, scientific explanation, on the other hand, requires very few, if any assumptions of new information beyond what we already know about sleep and the human brain, and even if it does (the most you have to assume is that those sleep processes are capable of screwing up in this particular way, which isn't much of a leap), they're things which can be relatively easily tested and confirmed.
Now, if a person's worldview already involves those things (demons, aliens, whatever assumptions are necessary) to make the alien/demonic/whatever theory more simple to them and requiring fewer new assumptions or questions, then sure, that would survive occam's razor, but personally, I'd say that's only true because that worldview itself probably
had failed occam's razor a long time ago, in assuming things about those creatures in the first place as the result of whatever forged that worldview to begin with.
Maybe not, but if I'm startled out of a dream into the half-awake state to observe what might or not actually be...but most definately appears to be some sort of creature feeding upon me, which then appears to flee when I attempt to fight it off...and if I then spend the rest of the day feeling drained and tired as if vital energies had been drained away...it's also no great leap of logic to suspect that maybe what appeared to happen...might be what actually happened.
Sure, except that the alternative explanations have more evidence for them. Yes, it's a great leap of logic to assume that an extremely implausible event that
happened when you weren't even really awake yet actually did happen for real, in spite of the lack of evidence against it, the evidence for alternative explanations, and how simple those other explanations are. If I have a dream about some kind of freaking anime character and wake up feeling like it really happened, it is not more reasonable for me to assume that anime characters are real and that I was meeting them in some other universe or something.
Ok. What if as a purely scientific experiment solely for the purpose of demonstrating how silly us people on the internet are...what if you were to in the privacy of your own home where none of us and no one you know would need to know about it...engage in corrective measures as proposed by some in this thread, such an prayer, mantras, or direct confrontation of your nightmare antagonists?
...and what if the nightmares ceased?
What would Occam's Razor have you conclude then?
It would have you conclude that meditative practices or other means of comforting yourself would prevent you from having nightmares, because this explanation is totally in line with what we understand about the human mind: That doing things to put you at ease about something
puts you at ease about something. It's damn near self-evident. Making yourself feel more at ease about your nightmares could very plausibly help not to have them or to be more in control of them, regardless of why the method supposedly works. Occam's razor would have you conclude that the method worked, but not that it worked because you actually beat up Bigfoot.