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Stress and diet having an impact on your health? Absolutely. The moon and light? That's bogus. Anyone who's cycle matches up with the moon over the course of their entire life is pure coincidence, but most won't. Even if you have a regular cycle, it can change over time (I'm 2 days shorter now than I was as 20 years ago).
@WadCheber Not from an evolutionary perspective, no. Laying down and getting back up for a horse is a liability. Horses and other herd animals (cows, goats, zebras, etc.) rely on being mobile in order to survive.
@Samuel How do you figure bending over a little is going to help? A human is going to need to be able to either tilt their head back about an extra 20 degrees or be able to turn their head almost completely around. Even if you argue that centaurs could have evolved to do this, it may be impossible to reach the teats with their flat faces. I've never seen a nursing mare's teats, but a non-nursing mare's teats are about the length and size of the first knuckle of your thumb.
In the movie Star Trek: Insurrection, the focus was on a group of people who settled on a planet that turned out to be a fountain of youth. Might be worth watching, as it does show some of their culture.