The wrong model of focus
Most people treat deep work like aerobic exercise: more is better, and the goal is to stay in the state as long as possible. So they sit at the laptop for four hours, get one good hour, and call it a day.
That is a misread of how the cognitive system actually works. Sustained attention is anaerobic, not aerobic. Like a 400m repeat. The system fatigues fast, recovers fast, and gets stronger with intervals — not with grinding.
The 90-minute block
The unit is one block of 90 minutes, structured as:
- 0–10 min — set-up. Open the file. Read the last paragraph you wrote. No new email, no Slack, no "just one check".
- 10–55 min — first work pulse. Single task. Notifications off. If you hit a dead-end, write the dead-end down in plain text and keep going.
- 55–70 min — break. Walk outside if possible. No phone, no screens. Drink water.
- 70–90 min — second work pulse. Pick up where you stopped. Push to a natural stopping point.
Two blocks a day is enough. Three blocks is excellent. Four is unsustainable.
What "focus" actually requires
Three conditions, in order:
- A clear next action — vague intent ("work on the proposal") burns the first 15 minutes
- No input — no Slack, no email, no news, no notifications, no incoming voice calls
- Physical readiness — fed, hydrated, low cortisol
Get those three right and the block runs itself. Miss any one of them and you will spend the block fighting the wrong battle.
Common mistakes
- Doing email first — primes the brain for reactive mode. Hard to switch out.
- Trying to "warm up" with easy work — the easy work expands to fill the time
- Pushing a third block when the first two were patchy — diminishing returns turns into negative returns
When to stop
The block ends at 90 minutes. Even if you feel hot, especially if you feel hot. The temptation to keep going is the same temptation that turns a hard training session into an injury.
The next block runs better when the previous one ended on schedule.
Steady wins because steady stops.