Tonight the city won't cool down. A hot night is the most unequal hour we have.

The heat had a daytime number today — around 37°C in southern England, on the Met Office's own forecast — and a daytime number is easy to talk about. It is the night that deserves your attention.

The Met Office has issued a rare red extreme heat warning for Wednesday and Thursday, with temperatures expected to reach 38°C, and alongside it a phrase worth sitting with: tropical nights. A tropical night is one where the temperature never falls below 20°C. They are forecast to be widespread this week, the forecasters say, especially in urban areas. The UK Health Security Agency has heat-health alerts running in parallel.

A hot afternoon is shared fairly. The sun is no respecter of postcode. A hot night is not shared at all.

If you live on the ground floor of a Victorian terrace with a through-draught, you will be uncomfortable and then you will sleep. If you live on the top floor of a 1960s block where the windows open four inches for safety and the brick has been soaking up heat since Sunday, you will lie awake in air that does not move. British homes were built to hold warmth in — that is the whole point of a British home — and in a heatwave that virtue becomes the problem. We have almost no domestic air conditioning, because for a century we never needed it.

So the tropical night sorts us. It sorts the shift worker trying to sleep through the day before a night on. It sorts the new parent. It sorts, above all, the old: the heat-health alerts exist precisely because a body over seventy struggles to shed heat, and a night that never cools is a night the body never gets to recover. The Met Office is blunt that consecutive tropical nights make it very hard for people to recover from the daytime heat — the danger compounds.

There is an infrastructure version of this too, and it is worth naming plainly rather than dramatising. The same forecasters flag impacts on power and water supply when a country this size all reaches for the same relief at once. A city does not have a volume knob. It has a grid, and reservoirs, and a finite number of cool rooms.

Here is where it lands on the question this paper keeps asking. The night is the part of the day we are not working — the hours that are supposed to be ours, for rest, for the people we live with, for nothing at all. A tropical night quietly confiscates that. It takes the one stretch of the day that was never about productivity and makes it, for the people in the hottest rooms, another thing to endure.

When the future-of-work conversation imagines lives with more free time, it pictures daylight: gardens, walks, long lunches. It rarely pictures the night going wrong. This week the night is the story. If we are going to talk seriously about what people do when they are not working, we should start with whether they can sleep — because the city that cannot cool down is telling us, in the dark, exactly who it protects.

Tonight, notice your own rooms. Then notice that not everyone's cool the same way.