I opened the window for fresh air and let the heavy air in

Half past eleven, and the thermometer outside has finally dropped below the reading indoors. That is the signal you have been waiting for, so you open the window.

For ten minutes it is wonderful. By one in the morning the room feels worse than it did before, the sheets are clammy, and you cannot work out what you did wrong.

You imported water.

The temperature fell. The water did not.

This is the whole of it. Overnight cooling lowers the temperature and leaves the dew point almost exactly where it was — because nothing removed any water from the air, it just got colder.

So a muggy evening becomes a muggy night. The outdoor relative humidity climbs towards 90% precisely because it is cooling, and that reading is not telling you the air got wetter. It is telling you the air got colder while staying just as wet.

Opening the window at that moment brings in air with a high dew point and mixes it into a room that may have had a lower one.

Why it felt good for ten minutes

Because moving air evaporates sweat from your skin faster, and that cools you directly, regardless of what the air is carrying.

That effect is instant and it is real. The humid air's effect on the room is slower — it takes twenty minutes to an hour to mix properly and raise the dew point of the space.

So the sequence is: immediate relief, then gradual deterioration, and the two feel unconnected.

The comparison to actually make

Dew points, not percentages. The dew point is a direct measure of the water present and is comparable between two places at different temperatures, which relative humidity is not.

Same temperature. Nearly twice the water. The thermometer cannot distinguish them and the dew point does it instantly.

What to do instead on a muggy night

Use a fan rather than the window. You get the evaporative cooling on your skin without exchanging the room's air for something wetter. This is usually the better trade.

Open up when the dew point genuinely drops. Often it does, later, if drier air moves in — and that shows on an hourly forecast as the dew point line stepping down rather than staying flat.

Accept the floor. Air cannot cool much below its own dew point, so a night with a 70°F dew point is not going to fall much below 70°F however long you wait. Waiting for a cool that is not coming is the trap.

In winter, open it freely. Cold outdoor air carries very little water whatever its percentage says, so ventilating in cold weather is powerfully drying. The advice reverses completely between seasons, which is why blanket rules about ventilation are so unhelpful.