I suppose it's like anything you get close to or delve into - there are details most people skim over or never have to encounter, examine, or inquire about. I find myself reaching into my coffee- or wine-tasting vocabulary, thinking of the various 'notes' of my fatigue/lethargy/weariness. Its volume, density, duration and lingering aftertaste.
In terms of other side effects, I'm definitely having aches and pains, but it is hard to differentiate them from normal. In a way they feel a little rawer, for lack of a term; they feel more immediate. Aspirin helps. I'm getting some muscle twitches and mild stiffness/soreness; I stretch like the cats every time I stand up. I may get motivated to actually take a yoga class if this keeps up. Mild nausea at bed-time which seems unrelated to anything I consume (or don't) so I'm wondering if it's the naltrexone clearing from my bloodstream (~13 hours from dosing would be about right) so I'll keep an eye on it as the dosage increases.
I've nodded off over the keyboard twice now, fwiw. Going to lie down with the cats for a bit.