Rooms are 100% sealed and co2'd. No smell or light gets out, ever.
Room walls are insulated with r13 fiberglass and a layer of r5 foam sheathing inside. Ceilings have 2 layers of r5 foam sheathing. It's soundproof as well.
Where I live it is hot as hell so sealing and cooling is mandatory. I ran nights for years but the buzzing of helicopters at night sketched me so I switched to days. MAYBE $15 difference from night running. (It's hard to pinpoint because rates went up, house AC is older, etc.
Cold I save a few $$$ running nights? Yes. But I sleep better this way :)