It may be placebo related.

And/or maybe it's an external cue.

I learned a cue from my dog. When she got into bed to go to sleep, she'd let out a deep exhale, like a sigh, which seemed like a signal that meant sleep time. I copied her, and it actually helps me to relax. It's a signal, an external cue, that produces a response in my nervous system.

Maybe the peppermint is a signal that produces a motor response. I've said many times that I wish we could harness the placebo effect somehow. Maybe you have found one way, an easy and pleasant one, too.