Great thing about the lights being off is the stomata are closed up tightly, for the most part.
You should avoid lower temperatures if you can. You go much below 18c and you fuck with biological processes. Depending on how much money you want to spend you may find avoiding extremes is 'enough.' Or, if you want to spend a bit more, reduce the difference between night and day further. This is reference info you can google for a more precise answer. I think to avoid 'extremes' you want less than a 10F difference? i'm not googling it, but you can find that type of reference info easily enough on your own as far as what to target if you want to tightly manage it.
Your RH spiked because temps dropped. If it showed absolute humidity, it would be a fairly flat line because transpiration is minimal at night - again, stomatas are mostly closed during this time frame and the steady absolute humidity is proof of that. The changing temperatures are the vast majority of the cause of RH oscillating in this particular graph.
What is causing a 10F drop at 1am? It looks like every hour your temp just drops 10F. That's a large range if caused by a space heater. At least around here, those mostly are a 4-5F range... temp drop 1F below, turns on until it reaches +3F being the standard for the lasko brand and shuts off. Should see a ~4F range oscillation result.
So, is this the equipment or is something cyclically happening in your house that you might be able to avoid its influence on the grow area -- like closing a door at night etc. if it is the equipment, there's not much you can do about the resulting range of temperatures.
In the end, i wouldn't throw too many resources at this. Avoid mid 60F and below at the least. If you don't mid teh extra expense, keep it closer to 20c or within whatever range you found while googling reference material. If 'they' say staying within 5F of daytime temps is optimal, do that etc..
If i recall it's better if vpd doesn't swing wildly either, but again with stomata closed it's less of an issue during lights-out cycle. Again, more reference info you should look up and not rely on user-submission. Go find a reputable source for the exact details... and a vpd table to help choose what RH to target relative to targe temperatures at night.