You forget one aspect. Light.
Actually, light is not the primary element in the governance of the human timeclock, insofar as not all parts of the earth (let's say portions of Alaska) have a standard light/darkness cycle. Some portions of the earth have several months of light and several months of darkness, for example. In this case, the natural biorhythms are more predominated by the adrenal cortex and an internal timeclock that has nothing to do with light/darkness.
People have a bio-rhythm and that is based on light.
Not always. Not all regions have the same light/darkness cycles as referenced above. Therefore, light/darkness cycles do not govern all human beings. Habituated adrenal programming becomes more of a primary factor.
Plants react to the length of the daytime, but animals as well.
Again, this is only partially true, being subject to regional differentiation. Further, various animals (and most plants in certain regions) enter into quite natural hibernation states. Unless you are infering that all animals sleep at night, which they do not. Nor can it necessarily be infered that plants sleep at night any more than animals do. Many animals sleep in the daytime.
If you change this unnaturally, you will suffer the consequences of it.
Not all change is unnatural. You are speaking from a vantage point of unfamiliarity, which thereby induces fear of the unknown. I believe that you may be confusing the natural with the familiar.
You cannot call this a natural biorhythm,
This is a natural biorhythm.
because 'force' yourself to be on a 48 hour cycle.
The practice would be near identical to a sports athlete in training. If you fear this, you will also fear a weightlifer or olympic runner who is enhancing their body through the science of exercise physiology. You judge the unfamiliar to be unnatural. Physiological adaptation is characteristically pain-associated as with common athletes. But this does not make it unnatural. It may simply be unfamiliar to you.
The planet is on a 24 hour cycle.......
No, it is not. The planet is on a 365 day cycle. Rather, you are on a 24-hour cycle. But you are not the earth.
you would need at least 20 hours of natural sunlight to get away with this without damaging you inner clock......
Your postulation fails to regard regional differences and variations of adaptive adrenal programming. You are asserting that your way is correct in all circumstances, when it is not thoroughly applicable.
or worse.....You're not evolving, you're forcing the unnatural
Evolution only occurs when the body and/or psyche are subjected to the unfamiliar. (in one sense, the 'unnatural', but we may be using the word differently here.) I believe that you may be confusing the natural with the familiar. They are two different things.