There's not much to write. I tried it for about two weeks, consistently, approximately no cheating: nothing but candles for lighting, no "screens", period (save, like, double-checking the alarm on my phone for the next day right before bed—but no browsing, no messaging, none of that, total use measured in seconds once the sun was down). No TVs or computers whatsoever. By night two I was already getting to sleep much earlier, and sleeping better. The effects of the experiment hit fast and strong.

You can still: read, write, play music, play card and board games, lift weights to some extent (I didn't—a bit contrary to the getting-tired, plus I just didn't keep it up long enough to build in anything like that, though nb you probably don't want to be attempting max-weight lifts in low lighting or otherwise doing anything that's not very-safe) or do many bodyweight exercises, take walks or run (night walks are nice!), talk, do limited chores or cleaning, plan meals (assuming written recipes, or books), and so on. There is a ton of stuff to do by candlelight, or in no lighting (including, ahem, partner activities). You won't run out of stuff to do if you've got any kind of varied tastes in activities at all—but you also won't be doing stuff so extremely engaging that it keeps you from noticing when you're tired.

If I'd kept it up, I'd have found a way to listen to "the radio" (but actually streaming or flacs or whatever) and had exceptions to the no-screens for, at least, setting that up initially. Goal would have been to still barely look at the screen, though: set an album or a few albums or a long playlist of podcast episodes or old radio show episodes (some of these are really fun!) to play, and mostly leave it alone. I'd also have probably made an exception for e-ink book reading devices, provided no cheating by using them to browse the web—that seems well within the spirit of the thing, to me, and unlikely to veer back into hyper-stimulative or way-too-bright territory.

Things I noticed:

As mentioned, full-room lighting is stupid-bright. Way brighter than needed, and that's an understatement. Made me recall some of my older relatives' long-owned houses and how many rooms didn't have built-in lighting, but just had a switch-controlled outlet with a lamp attached—even just having the light come from somewhere lower made the room less intensely-bright, all else being equal. That seems far saner to me now, than what's normal in new houses. Are you trying to take daytime-like photos indoors at night? No? Then your lights are probably way too bright, likely by an order of magnitude, than what's remotely necessary. There's no way that's not affecting sleep—right?

You do need to have a way to carry one or more candles. You don't want to have dedicated candles lit in every room (including bathrooms!) unless, I dunno, you have people over maybe. Could probably safely remove a lot of this need with a small number of very-dim night lights (some are way brighter than a couple candles, though! Gotta shop carefully) without ruining the whole exercise.

Candles are painfully bright if they're your only light source, they're nearby (say, you're reading), and you look right at them, even in passing. Very unpleasant. If I'd kept it up, I'd have had to find a way to shade them somehow.

In fact, though, to keep it up longer-term, I'd have had to find or build an electric alternative. Unscented candles that don't smell like absolute ass (once you've smelled anything other than the common petroleum-based ones, those smell like a chemical waste dump) are fairly expensive to burn, are a fire hazard (obviously), and harm air quality even when they're a relatively-decent material. Most e.g. portable camping lights are far too bright and are a too-cold temperature to replicate candlelight. I expect I'd have ended up building a few little battery-powered lanterns with the dimmest warm-temp LEDs I could find, or maybe even dim and under-driven filament bulbs to get a smoother light curve—they'll last a long time if you run them way under-spec, and the inefficiency hardly matters at powers that low.

One persistent change I've made as a result of this is having a very-dim-bulb warm-temp floor lamp (some cheap Ikea thing, with about the dimmest warm-temp LED bulb I could find that'd fit it) for my office. It's the only nighttime lighting I use in there, most of the time—rarely use my overhead light. Point it at the wall if I just want to avoid stubbing my toes on things, aim it almost at me if I'm reading. It's more than enough light. Been meaning to swap out the end-table lamp bulbs in the bedroom with something about 1/4 as bright, but keep forgetting.

Another thing is that the past is a little more relatable. I've lived with candles (if not long—but it's enough to notice some things!) and know the pain points and really get and appreciate tools & design features that were built with them (and oil lamps, rushes, all that stuff) in mind, in a way I didn't quite so well before. Stuff uncommon now that used to be somewhat-common, like family read-alouds of novels make more sense to me now (not only were books more expensive, so you might have fewer of them—it takes way less lighting for one person to read, than for five, which really matters when light is expensive! IIRC the Brontës were supposed to have done a lot of this, but I gather many people did)

What makes it hard to keep up is life. In the Winter, especially, even if I'd tried to keep doing this, I'd have had to set a cut-off later than sundown, no question—you can't realistically go no-electronics-or-lights at like 5:30PM. Nobody else lives like this. (Almost) no place you go with others at night is like this. Chores pile up during the day because stay-at-home spouses aren't the norm anymore, so by the time you have even a second of free time it's already night, or else you've deferred all the chores to night, and adding friction to those is not fun. Going out after dark anywhere (say, grocery shopping) kinda screws it up. It's just fairly inconvenient given that all of life is now built around having day-like lighting available at night.

However, I think getting a good chunk of the way toward it is feasible, and would probably bring many of the same benefits.