I thought perhaps there could be genes that would effect how quickly a beast regains energy. For example, the positive one could be "fast metabolism", where the beast would earn energy 25% more quickly, but eat twice as much as a creature with normal metabolism. Or creatures with slow metabolism could earn energy half as quickly, but eat a quarter of as much as normal-metabolism beasts. 

Figured it was worth putting the idea out there, at least - I think this way people could breed/pick creatures to better fit their personal time cycles, but still keep things balanced, maybe?

At first I was like that is going to eat up all my food. But if the slow metablism ones eat half as much food that would balance it out.  I really like this idea, but you would have to add another dodad to it, i.e. Bones, Teeth... etc.

I was imagining it as being another type of "organs" gene, as I imagine that would be much easier to implement that way. It's nice to hear that someone else is interested in the idea :)

I think it should be added to the organs gene to, because all my pets that are already fully reasearched would have to be researched for another 2 vermillion and 200 coins.  Pretty cheap then but doing it for a bunch adds up to be really expensive.  Yodra, I really like the idea. It is GENIUS!

Ah, thank you so much, I'm really flattered to hear that! The cost of researching if it was implemented separately is a good point, and honestly I think a whole new category for just two or three genes would be kind of silly. Besides which, metabolism really is a function of your organs, so I think it could fit well enough with the genes already present.