In principle I should now perform proper scientific trials, removing each intervention in turn to assess effectiveness, but the experimental subject currently reports vastly improved quality of life and is refusing to sign the waiver

Same situation for me, though not sleep-related. Made several changes, don't know which worked, but improved quality of life ...

For me it is the mask.

And the answer could be any combination, including all three.

As a life long insomniac, I approve of this message

I suspect you'll eventually find out by way of unplanned experimentation, like, life getting in the way, travel etc. If I say you'll find out "pretty soon", does that make me an optimist or a pessimist? 😜

For me, using a noise machine was the single biggest improvement. Like it so much I bought a second for travel!

And it might turn out after all that that none of them work unless you do all three. 😀

We're funny that way, huhn.

You never need to test removal of #3 because there is no combination that doesn't benefit greatly from #3. May I also further complicate your routine by introducing #4: foam ear plugs.

Ok, time for Design of Experiments so you don't have to eliminate interventions entirely.... Half fill the humidifier one night. Cover one eye with the sleep mask another. Write in the journal, but limit yourself to quips on a third.