Insulin response and other metabolic factors seem currently not measurable non-invasively, like EEG. That's why I was focusing on muscle tone as an easily available parameter which can be measured continuously and non-invasively, in line with EEG. Temperature is another similar parameter and I've seen a couple of studies showing reduced frontal lobes' temperature promotes sleep.

It can seem tricky to reduce it without pharma or techniques performed while awake, but I've collected a corpus of research which may allow to emulate the action of e.g. GABA without adding the actual GABA (which is thought to induce muscle paralysis in sleep), thus non-invasively promoting the reduction of skeletal muscle tone.