Kirill, The Zero Fog · Not a doctor · 6 September 2026

Why do I get tired after lunch?

Lunch is the smaller of the two things going on in the early afternoon, because a dip in alertness arrives with or without food and a big lunch makes it deeper.

For years I was sure it was the lunch. I'd eat at one, and by two the afternoon was gone, so I went after the lunch. I ate less of it, then none of it, then salads instead of sandwiches and chicken instead of pasta, and for a while I walked straight after. Each of those got a proper go, and every time it ended the same way, with me at the desk at half past two reading the same email for the third time.

I'm not a doctor and none of this is medical advice. When I typed the question into Google, what came back was a clinic page about food comas and a lot of pages about blood sugar, and none of it matched my afternoons.

While I was reading for this I counted. Of roughly 8,800 people whose posts I went through, eleven wrote "after lunch" and nobody wrote "food coma". That word lives on clinic pages. People write "after lunch" and "my lunch break" and leave it there.

Why do I feel extremely fatigued after lunch?

Lunch deepens a dip that arrives every afternoon anyway, and the bigger the lunch the deeper it goes.

A driving study from Loughborough, published in 2012, is the one that made sense to me. Twelve young men drove a simulator for two hours after lunch, once on about 300 calories and once on about 900, two lunches built to look and taste alike with the heavy one carrying three times the fat, and none of the twelve started the drive fresh. On the heavy lunch they drifted out of their lane more often. But for at least the first half hour the two lunches looked the same, and under both of them, light one included, the drive got worse as it went on. The authors themselves call the dip largely unrelated to lunch.

I'd been eating the heavy one for years.

What's in the lunch is less clear, and the Loughborough authors call that literature mixed:

• Reading, 18 people. Three lunches of equal calories, and the one closest to what they normally ate came out best, while more fat than usual or more carbs than usual both slowed their reactions and left them rating themselves muddled.
• Sheffield, 8 men. Equal calories and equal protein, and the fattier lunch left them less alert and slower on a task that needed steady attention.
• Pittsburgh, 12 people. Lunch was replaced with hourly liquid feeds, and five of the twelve dipped in the early afternoon anyway.

My experiments were the unusual lunches on purpose, and the Sheffield task is the one I was failing at every day. The Pittsburgh study is the spine of why I was crashing at 2 PM every day.

These are small studies, eight to eighteen people each, so I only take the direction from them. A big lunch makes it worse and an unusual one probably a bit worse, so the best lunch I found was a lighter one of whatever I usually ate. But the same twelve men who did better after the light lunch still got worse over the two hours.

Why do I have difficulty concentrating after eating?

A meal hits one particular skill, the ability to keep attention on one thing and react fast when something changes, and that skill is most of desk work.

Smith and Miles measured this in 1986. After lunch people were worse at holding attention and reacting quickly to a new signal, resisting distraction wasn't affected, and things like movement speed changed with the time of day and weren't touched by the meal. That was my afternoon. I'd notice every Slack ping and not get to the end of a paragraph.

A PhD student posted that after lunch they got too foggy to work on their paper, and they put a number on it, about four hours lost every day. Dozens of people answered, and a lot of the answers were a different lunch: skip it, cut the carbs, move the calories to dinner, walk afterwards, get your blood sugar checked. Two people suggested prescription stimulants. One reply said a bit of fog after lunch is normal but four hours a day is something else, and pointed them at a neurologist. Four hours a day was my number too, and I'd already tried most of that thread.

Why do I feel brain fog after every meal?

Fog after every meal, breakfast and dinner included, is a different pattern from fog after lunch alone, and it's the one I'd take to a doctor first.

Most pages on this question will tell you it's blood sugar, and it might be. Mine came back clean, and I'm not going to tell you what yours means. A second thread was from a student eating two cups of rice at every meal who lost an hour after each one, three or four times a day. A different shape from mine, and one I'd want a doctor to look at. (Most of the replies told them to eat less rice, which they'd probably worked out.)

Mine was only ever lunch. Breakfast and dinner never did anything to me. The meal at the start of the afternoon was the only one followed by a wall, and when I skipped it the wall came anyway and I was hungry as well.

What a clean set of tests rules out, and how often a cause turns up in someone who comes in tired, is its own piece: normal blood test but still tired.

What I changed about lunch, and what it did

Two lunch changes stayed, a smaller lunch and ten minutes outside, and both made the afternoon easier without making the dip go away.

A smaller lunch, the food I always ate and less of it. The dip was shallower and I could sometimes work through to three instead of stopping at two. I was doing that before I'd read the Loughborough paper, and it matched.

Ten minutes outside at the first sign of the dip, without the phone. The dip came anyway, and the first hour of it was easier to get through. I still do it.

I stopped the rest, the skipped lunches and the salads and the chicken. Every one of them assumed the meal was the cause, and the studies say it's the amplifier. What I still don't know is why it hit me so much harder than other people, who eat what I eat and are fine at three.

Where I ended up

I eat the lunch I always did and I work a full day now. I stopped guessing, ignored the gurus and stuck to doctors who see patients all day and say what they actually see. What I found had nothing clever about it. No secret technique, nothing to swallow, no schedule to keep. There's still something left in the evening for my family and the people close to me.

I put it into a free workshop for people who work from home. It's about what actually runs out, and why it was never discipline. There's nothing to buy on the way in. If you've tried as many lunches as I have, join the free workshop.