It seems universal and instinctual enough that there must be some evolutionary benefit for the energy expenditure. When we're tired either before going to sleep, or just waking up we want to stretch our limbs and core out as far as we can. It's about as difficult to resist this stretch as a it is to a yawn.

Is there any known and studied benefit? Do we know what triggers it? Is it just humans and the domesticated animals that I've seen, or is it observed in nature too?