Look, an extraterrestrial!

Like other mammals and, to varying extents, most living things on Earth, humans have a circadian rhythm: a natural, hardwired internal clock that regulates metabolism and other operations of body and mind. When exposed to the cycle of sunlight and dark, this rhythm keeps approximately in time with a day. When humans live in places isolated from these natural cues (for instance, on extended tours of duty on a submarine), they usually, for convenience, create articial rhythms of light and dark on the same 24-hour cycle.

However, when a human is left free of any external cues of light and dark, an interesting thing happens. If he has nothing to calibrate his internal cycle with, it will naturally, and fairly quickly, settle into a rhythm of not 24 hours but 25.

On an apparently unrelated subject, there are a few oddities about how the human musculoskeletal system works, which are often pointed to as evidence that the evolution from quadruped to biped hasn't quite finished. However, some people point out another odd observation: on a planet whose gravity was half of Earth's, our musculoskeletal system would actually work pretty darned good.

Hmmm... we seem to have an innate biological prejudice for low-gravity worlds with 25-hour rotational periods. It's almost as if we come from somewhere else nearby...

Okay, while this is intriguing, I'm by no means serious. This is great fodder for a surrealistic roleplaying game, but certainly not anything real. Though, hmmm...