Read the back of almost any calming chew and melatonin is near the top of the list. There is a reason for that, and it is worth understanding before you buy another bag.
What melatonin does
Melatonin is a hormone involved in signalling sleep. It tells the body that it is night. In pets it is widely used, generally considered low-risk at appropriate doses, and it produces an effect an owner can see within about an hour: the animal gets sleepy.
Why that isn't the same as calm
Sleepiness and settledness are different states. A sleep signal pushes a tired animal over the edge into sleep. It does not lower an elevated stress baseline — so the underlying chemistry is still there when they wake up.
This is why so many owners describe the same experience: the evenings went quiet, but the reaction to the mail truck at 2pm never changed. The product worked on the hours you were watching, not on the problem.
Why it dominates the shelf anyway
From a commercial standpoint melatonin is close to ideal. It is inexpensive, it is legal, and it produces a visible effect fast enough that the buyer concludes it worked. A product does not have to change anything to feel like it is changing something.
That is not a conspiracy. It is an industry optimising for what is easy to sell rather than what is hard to do.
What to look for instead
If your goal is a pet who settles on their own rather than one who sleeps on command, look for ingredients studied for supporting relaxation without sedation — L-theanine, chamomile, passionflower — at doses that are actually printed on the label rather than hidden in a proprietary blend.
And judge the product on a full month, not an evening.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. Consult your veterinarian before giving any supplement, particularly if your pet takes medication.