The Zeta physiology differs from the human physiology in several respects, and this has created challenges and choices.
It has been noted that where we have nostrils, and our breath, acrid by human standards, can be felt when in close quarters, we don't appear to be breathing. The expansion of human lungs is visible, yet our trunks seem firm and immobile. Do we have lungs? We do indeed, but they work asynchronously, rather than in unison, one expanding while the other exhales, so the motion is all internal. How did this come about, that springing from the same base we developed different breathing patterns? The answer lies in our long lean frame, which in the trunk has a fused rib system with cartilage between the ribs for extra strength. During our genetic engineering periods those Zetas who adapted to a longer more rigid frame by an adjusted breathing cycle survived to pass along their genetics. In fact, our current frame only became possible because asynchronous breathing became possible. The genetic engineers went with the flow, so to speak. Our hybrids, however, have unison breathing, but in a pinch could switch over to asynchronous breathing - a latent capacity.
Where our current digestive system is rudimentary, having been eliminated in preference to a cleaner, quicker form of nourishment intake via our nutrient baths, the genetics controlling digestion are a factor to be considered when merging genetics in the hybrids. What did our digestive systems used to be, and how did these differ? The human diet is voluminous and fibrous, reflecting the diet of the base apes that formed the human race, and in truth reflecting the reality of vegetation on your planet and the adaptation that animals evolving on your planet were forced to deal with. If an animal is vegetarian, and most are, they eat continuously, defecate frequently, and deal with digestive gasses. Few animals become successful predators, an obvious fact as if all were predators there would soon be nothing to prey upon. So humans have as their base an animal that eats masses of fibrous vegetation, daily.
We Zetas, on our world, had a different diet. Our world was not sunny and bright, and the vegetation thus not as evolved. Mosses, bugs, algae, crustaceans, and when we ate almost nothing went to waste. If forced to eat your diet we would digest the fiber, but would also be unable to consume enough to retrieve the necessary nutrition. In our hybrids we have attempted to take the best of both worlds. Humans, we feel sure, would not object to less farting and burping. We have retained the more complete digestive capabilities of the Zetas, while adjusting this to allow for the types of foods your world offers. In the highly civilized world of the future, cultivation will focus on those foods that return the highest nutrition. Less eating, less elimination, virtually no digestive gasses, and surprisingly constipation will not be a concern as the shape of the bowel, being a single straight shoot, lends itself to guaranteed elimination.
The physiological work assigned to organs such as the kidney and liver is essentially the same in both Zetas and humans, but how this work is accomplished varies. In humans the kidney works constantly, filtering waste, but in the Zeta click in only when certain blood chemicals raise to a level requiring attention. Like the heart, which rests between beats, the Zeta kidney rests between Zeta meals. The reason for this pacing is the more complete absorption of nutrients in the Zeta digestive tract, placing a burden on the kidneys which then require a rest period in which to recoup. Likewise, the Zeta liver is a workhorse that goes the distance after meals and requires a recuperative period. Zetas who overeat get more than overweight, they get sick. Our thinness is not due solely to genetic tendencies, we simply do not overeat. Do the Zetas store fat as humans do? Yes and no. We have this capacity but our physiology does not make use of it ordinarily. Since we do not digest excess food without getting sick, our bodies keep all nutrition at the ready, as a store to be used up before the next meal. No packing it away in fat cells, no saving it for a later day. Our hybrids have the best of both worlds. They are slender because their physiology uses, rather than stores nutrition, and their appetites pace accordingly. But should they for any reason decide to overeat, they do not suffer as we, the Zetas, do.
The Zeta were selected for production of a hybrid race with humans in part because our physiologies are similar. Our blood is also hemoglobin based, and where a chemist would find all the many Zeta blood components differing from comparable human blood components in quantity and composition, the differences are not dramatic. Likewise with the endocrine systems, which have comparable glands and functions. Here, however, there is one dramatic difference. The Zeta do not sleep, though there are regular rest periods where activity essentially stops. This difference appeared in the Zetas during our 3rd Density existence, when many of our number were warlike. The guard who did not sleep survived, and thus the trait became dominant. There are blood chemistry differences in humans between the wakeful and sleep states, but in the Zeta these differences do not appear. Our hybrids sleep, as we have found the physiology more resilient when able to rest. However, the sleep requirements are much reduced, requiring perhaps 1/5, rather than 1/3 of the day, and the onset of sleep deprivation does not occur with our hybrids, who are able to bear through without sleep if need be.