I don't believe there is any evidence that you develop a tolerance. As a side note, Melatonin changed my life.

The body will develop a tolerance (or the reverse, a sensitization reaction) to just about any substance that's taken regularly due to homeostasis. However, so long as you're taking a dose that nudges some physiologic signal/need in the right direction, your body's response to the substance will be minimal.

I'd be surprised if you can find anything this isn't true for.

A lack of studies on what the tolerance looks like for a particular substance does not imply that tolerance does not form.

In the case of melatonin: It's almost universally true that your sleep quality is worse than it was before once you stop taking it (for a few days at least). That's an indication that your body's equilibrium has changed from habitual use.

Melatonin does have the potential for augmentation, but it isn’t a certainty or even the majority of people.

Oddly, the result isn’t a loss of initial efficacy but instead a ‘wide awake at 3am’ situation.

Not in my case. Never had this sleep quality. But maybe it's placebo after a few weeks? :)