Bedroom particulate reading climbs every night from about 11pm and nobody is cooking

Two things are probably happening at once. There genuinely is a source - bedding, skin, and any movement stirring settled dust off soft surfaces, all in a small sealed volume with no air changes to carry it away. But cheap optical sensors also read high as humidity rises, because particles absorb water and grow, and a closed bedroom at 3am with two people breathing in it is the most humid the room gets all day. Easy test: run the same monitor overnight in an empty closed room. If the line stays flat, your particles are you; if it still climbs, you are watching the humidity curve wearing a costume.

I chased this for about a month before I found it, and it was the humidifier. Filling it from the tap meant it was quietly aerosolising dissolved minerals all night, and my monitor was counting them faithfully. Switched to distilled and the overnight line flattened out completely. Worth checking whether anything in the room is running only at night.

I put two different monitors on the same nightstand for a week out of stubbornness. They agreed during the day and diverged overnight by a lot, the cheaper one always reading higher as the room cooled. That does not mean nothing is happening, it means the absolute number overnight is not something to make decisions on. What I trust now is the shape of the curve on one device over time, not the value.

Different suspect: outdoors. In winter my overnight peaks lined up almost exactly with neighbours' wood burners, and the reason it fell in the morning was that opening the door mixed the bedroom with the rest of a bigger, cleaner house rather than that I had ventilated anything. Check the timing against an outdoor station near you before assuming the source is in the room.

Before spending anything, try one night with the bedroom door open and one with it shut, everything else identical. If the peak disappears with the door open, the answer is air changes and the fix is ventilation rather than filtration.