Around 3pm, the meeting gets heavy. You blame the lunch. It is usually the air.
Here is what actually happens over a workday. People file into a room, the door closes, and everyone starts breathing out CO2. There is nowhere for it to go. So it climbs. By mid-afternoon, a busy meeting room that started near 500 ppm is often sitting at 1,500 to 2,500 ppm.
That is not a small thing. Fresh outdoor air is around 400 ppm. Above about 1,000, most people start to feel it: foggy, slow, one eye on the clock. It arrives at exactly the moment you are trying to push through the afternoon.
Harvard researchers put people in rooms at different CO2 levels and ran cognitive tests. As the CO2 climbed, the scores dropped, across decision making, using information, and strategic thinking. At the levels found in normal, busy meeting rooms, some scores fell by close to half. Separate studies link higher CO2 to slower reaction times and more small errors.
So the "3pm slump" is not just tiredness. Part of it is a measurable load on your brain, delivered by a room that has quietly filled up with the gas you exhale.
Here is how to prove it to yourself. Next time the room feels heavy, step outside or open a window. Within a few minutes you feel sharper. That is not a placebo. That is the CO2 dropping back toward fresh.
Open a window or prop the door. Take a real break every 45 to 60 minutes and air the room out. Do not cram eight people into a room built for four. And if it is a room you do not control, measure it, screenshot the worst reading, and take it to whoever runs the building.