There’s exhaustion, which builds up and is relieved gradually with sleep, and is effectively masked by caffeine. Then there’s the feeling of incomplete/interrupted sleep; maybe the quality of sleep was poor somehow, maybe you were woken up by an alarm in the middle of a sleep cycle, but it’s painful, cognitively debilitating, and sticks around like a splinter in your brain. Caffeine doesn’t help it at all, but it can be completely relieved by a nap in which you succeed in falling completely asleep even just for a few minutes, which caffeine does not prevent from happening.
I don’t know how much of that is objectively how it works for everyone, but that’s how I understand it.
But is chicken-ness actually defined by genetics? An important characteristic of a chicken is its domesticated status, if you consider the birds they descend from, they are remarkably similar, and it’s hard to imagine that any one mutation would have been what caused people to start calling them by their own name or considering them as a separate species. It’s possible that the first chicken became the first chicken when it was captured by humans, and so preceded the first chicken egg.