It is not always tiredness. Sometimes it is the air.

I promised that we would return to air. It is the subject I have come to like most — because nothing in it is visible, yet it changes everything.

Think of an afternoon in a closed conference room. Or a bedroom in the morning, after the window has been shut all night. That heavy-headed moment when thoughts move more slowly. We usually tell ourselves, “I am tired.” Yet often it is not tiredness — it is the air.

Here science tells us something that surprised me. Researchers at Harvard University carried out the COGfx experiment: the same people worked in rooms with different air quality, without knowing which was which. In better-ventilated rooms with lower carbon-dioxide levels, their results in tests of thinking — planning, decision-making and responding to a crisis — were markedly higher. The same mind. Different air. A different result.

This is precisely the two-way relationship I wrote about at the beginning: a room does not merely contain us. It helps determine how we think inside it.

And the good news is that air is the easiest thing to improve immediately. No renovation is required. Air rooms briefly but often, do not keep the bedroom sealed all night, and bring a few plants into the home. Small habits whose effect you can feel the very same day.

Today’s question: is airing the rooms a habit for you — something you do every day without thinking — or do you remember it only when the air has become heavy?

Next Thursday: noise. The kind we stop hearing, yet which still tires us.