The 90-Minute Myth: Why I Plan My Day in 25-Minute Blocks

Most productivity advice tells you to block out 90 minutes to 4 hours for deep work. I think that's wrong for most people. Cal Newport, who popularized time blocking, does recommend protecting 90-minute to 4-hour blocks for cognitively demanding work (Time blocking (Wikipedia)). But unless you're a tenured professor with a closed office door, that advice will break you. I plan my day in 25-minute units, and I think you should too.

I'm writing this for the person who has tried time blocking and failed, who has downloaded every task app and still ends the day wondering where the hours went. You don't need a new app. You need a plan that matches how your attention actually works.

1. Start with the night before, not the morning

I used to plan my day at 8 a.m., coffee in hand, feeling virtuous. It never worked. By the time I finished planning, I had already burned my best cognitive window on administrative work. Now I plan the night before, and I use the Ivy Lee Method to do it.

The Ivy Lee Method is simple: at the end of each workday, write down the six most important things to accomplish tomorrow, prioritize them in order of true importance, then the next day work only on the first task until it is finished before moving to the second, carrying any unfinished items onto a new list of six (The Ivy Lee Method (James Clear)). There's a famous story that Charles M. Schwab paid Ivy Lee $25,000 for this advice in 1918, but no contemporaneous primary documentation of that meeting or check has been confirmed (Talk:Ivy Lee (Wikipedia)). I don't care about the story. I care that the method forces me to make decisions when I'm tired, not when I'm fresh.

This matters because of decision fatigue. Vohs, Baumeister and colleagues found across four laboratory studies and a field study that making many choices depletes the resource used for self-control, leading to less physical stamina, reduced persistence, more procrastination, and worse arithmetic performance (Vohs 2008 choice and self-control (PubMed)). If I plan tomorrow's day at 5 p.m., I've already made hundreds of decisions. But if I plan tomorrow's day at 5 p.m. for tomorrow, I'm not trying to execute those decisions while depleted. I'm just writing a list.

2. Turn those six tasks into 25-minute appointments

Here's where I break with Newport. He says block 90 minutes to 4 hours for deep work. I say block 25 minutes. The Pomodoro Technique, developed by Francesco Cirillo in the late 1980s, breaks work into 25-minute focus sessions with 5-minute breaks, and a longer 15 to 30-minute break after four sessions (Pomodoro Technique (Wikipedia)).

I take my six tasks from the night before and I estimate how many Pomodoros each will take. Then I put them on my calendar as 25-minute blocks. A task that I think will take two hours becomes four or five Pomodoros. This does two things. First, it forces me to confront the planning fallacy. Kahneman and Tversky named the planning fallacy in 1979: people underestimate how long a task will take even when they know how long similar tasks took in the past (Planning Fallacy: An Inside View (SPSP)). When I have to assign a number of Pomodoros, I can't hide behind vague optimism.

Second, it uses Parkinson's Law. Cyril Northcote Parkinson wrote in a 1955 essay for The Economist that work expands so as to fill the time available for its completion (Parkinson's Law (Wikipedia)). If I give a task 90 minutes, it will take 90 minutes. If I give it 25 minutes and a hard stop, it will often take 25 minutes.

3. Take 90-second breaks, not 10-minute ones

This is the part most people get wrong. They take a 10-minute break every 45 minutes, check their phone, and wonder why they feel worse. A 2025 Frontiers in Psychology study of 253 second-year undergraduates found that a seminar with 90-second micro-breaks every 10 minutes produced better end-of-session quiz performance (65.13% average) than a traditional single 10-minute break 45 minutes into the seminar (56.44% average) (Micro-breaks study (Frontiers in Psychology)).

I don't take 5-minute Pomodoro breaks to check email. I stand up, walk to the window, and look outside for 90 seconds. That's it. The same study found that human attention is limited to about 25 minutes of sustained task performance, with declines reported after as little as 10 minutes (Micro-breaks study (Frontiers in Psychology)). So I don't try to push through. I reset.

4. Protect the first 90 minutes of your day for your hardest task

I don't care if you're a morning lark or an evening owl. I care that you know which you are. Research shows a synchrony effect: people perform best on challenging cognitive tasks when the time of day matches their chronotype. Early chronotypes peak early, while late chronotypes perform best in the late afternoon or evening (Best times for cognitive functioning (Frontiers/PMC)).

Facer-Childs and colleagues found that early chronotypes showed only about 7.6% variation in performance across the day, while late chronotypes showed about 26.2% variation and were significantly impaired on psychomotor vigilance, executive function, and physical tasks in the morning (Time of day and chronotype (Sports Medicine Open)). If you're a late chronotype, stop trying to do deep work at 7 a.m. You're not lazy. You're fighting your biology.

I'm a morning person. My first 90 minutes are for the hardest thing on my list, usually writing. I don't check email until I've finished at least three Pomodoros. That's my rule.

5. What can go wrong: the interruption spiral

Here's the warning. Your plan will collapse the first time someone interrupts you. Gloria Mark's field research at UC Irvine found it takes an average of 23 minutes and 15 seconds to fully regain focus after a single interruption, and that workers switch activities or are interrupted on average every 3 minutes and 5 seconds (UC Irvine: Mark on interruptions).

If you get interrupted during a Pomodoro, the Pomodoro is dead. Cirillo's method is clear: a pomodoro is an indivisible 25-minute work interval; if it is interrupted, it is abandoned (Pomodoro Technique (WVU Online)). Most people try to salvage the interval. Don't. Mark it as abandoned, take your break, and start a new one. The alternative is attention residue. Sophie Leroy's 2009 study showed that unfinished tasks leave attention residue that impairs performance on the next task (Leroy 2009 attention residue (IDEAS/RePEc)).

I also don't multitask. The American Psychological Association found that switching between tasks can cost up to 40% of productive time (APA: Multitasking - switching costs). And only about 2.5% of people can genuinely handle two cognitively demanding tasks simultaneously without performance declining (2.5% multitaskers (Charlotte Observer)). I am not in that 2.5%. Neither are you.

Sources

• Time management methods (Wikipedia) - https://en.wikipedia.org/wiki/Time_management
• Pomodoro Technique (WVU Online) - https://online.wvu.edu/news-and-stories/2026/03/19/the-pomodoro-technique-helping-online-students-get-things-done
• Micro-breaks study (Frontiers in Psychology) - https://www.frontiersin.org/journals/psychology/articles/10.3389/fpsyg.2025.1589411/
• UC Irvine: Mark on interruptions - https://ics.uci.edu/2017/01/30/medium-bad-habits-you-need-to-kill-immediately-to-be-a-much-better-person-this-year-mark-cited/
• APA: Multitasking - switching costs - https://www.apa.org/topics/research/multitasking