Many Taoist-oriented meditators, martial artists, and psychics say it isn't a good idea to practice during thunderstorms, typhoons, etc. You make yourself vulnerable to harmful energies, or energy that is just too wild for you to incorporate. Also, it is supposedly nearly impossible to concentrate your will and energy in a high wind. My attempts at doing qigong on the beach corroborate this claim. Just doesn't seem to work.
This might extend to OOBEs as well. Wind and storms penetrate through many energy levels; their medium is not just "physical" air. Might not be a good idea to try OOBing during a thunderstorm, although then again, you might have a very memorable experience.
Other Energy Structures
From what I understand, the astral topology is partly shaped by energy structures deriving from plants, human constructs, and geographical features. I think Charles Goldman is right that some of the astral topology has been orphaned from its original physical substrate; other astral features probably never did have a physical substrate. Another thing to consider is that the astral world is not just a projection of the physical; rather, both are parts of a coherent whole. This means that such things as the extinction of languages, cultures, ideas, and species has analogs on etheric, astral, intentional, and other energy levels. Or maybe there are physical entities that are orphaned as well -- bereft of their astral components. I won't mention any names! ha ha ha.
Oh yeah, one other thing. They say here that if you meditate under a big tree, it can suck away your energy. Usually when you meditate, you absorb energy from the sky, earth, etc., but trees are much more powerful magnets for this energy than we are. That's why they don't have to eat. The potential energy differential between human and tree creates a flux that goes the opposite way you want it too! So what you have to do is respectfully ask the tree if you can use some of the energy that is rightfully its. "Rightfully," as in "might makes right"! A 500-year-old tree is extremely powerful. It sucks in tremendous quantities of energy, and if you open up your barriers to energy exchange, as happens in meditation, it can suck some of your energy right out of you. The formula I learned is to pat the tree three times while silently asking it to use the energy. What if the tree says NO? Does this ever happen? Could you tell if it did? Beats me.
Actually, this issue isn't so related to OOBEs anymore (sorry), because the OOBer is not in such a state of open energy flow, as far as I know. I'd be really interested in any other dimensions to tree energy from shamanic or Western paranormal perspectives.
One reason why metabolism changes when you start OOBing or meditating is that you give your personality a rest. It takes a lot of energy to carry that big heavy thing around in front of you all the time. Putting it down frees up a lot of energy.
Another factor is that through encountering and dealing with unconscious hang-ups, fears, and other energy stagnation, you reduce impedence. It's like if you scrape all the cholesterol off your arteries, your heart won't have to work as hard to pump the blood through. In qigong meditation, you can even feel blockages being dissolved. Various parts of your body go cold, hot, feel "full", tingle, etc. This is the sensation of energy flowing through long-unused passageways.
Actually, the above two factors really amount to the same thing, since you could look at your personality as a pattern of knots, blockages, coagulations, etc. that has become stiff with time. That doesn't seem like that's all there is to it though. Depends what you mean by "personality." Does seem to make some sense though, doesn't it?
If anyone wants to respond to any of this stuff, just email me.