Ninety minutes is a popular length for a deep work session, and the reason usually given is the ultradian rhythm: a cycle of roughly 90 minutes that runs through the body day and night. The idea is real, the evidence for it during the day is mixed, and 90 minutes turns out to be a good block length anyway. Here's what we know.
What an ultradian rhythm is
Rhythms shorter than a day are called ultradian (circadian rhythms run about a day). The best-known is the sleep cycle: through the night, sleep moves from light sleep into deep sleep and then REM in cycles of about 90 minutes. In the 1960s, the sleep researcher Nathaniel Kleitman, who co-discovered REM sleep, proposed that this cycle doesn't switch off when we wake. He called it the basic rest-activity cycle: during the day, too, alertness would rise and fall roughly every 90 minutes.
What the evidence says
Some studies have found daytime fluctuations in alertness, daydreaming and even hunger on cycles of around 90 minutes; others have found the rhythms weak, irregular, or swamped by the bigger circadian swing across the day. It's fair to say there's no clock in your head that reliably ends your focus at minute 90.
A separate line of research points to the same number for a different reason. When Anders Ericsson and colleagues studied violin students in Berlin in the early 1990s, they found the best players practised in sessions of around 60 to 90 minutes, with breaks in between, and rarely more than about four hours of deliberate practice a day. Hard, focused work seems to have a natural ceiling, whatever the mechanism.
Why 90 minutes works as a block
- It's long enough to get past the warm-up. Loading a hard problem into your head takes ten or fifteen minutes; a 25-minute session spends half its length getting started.
- It's short enough to stay sharp. Most people can hold real focus for an hour and a half; few can hold it for three.
- It divides a day neatly. Two or three 90-minute blocks, with breaks, is a full day of deep work, which is about what experts manage.
How to work in 90-minute blocks
- Book them, preferably at your best hours. The chronotype quiz suggests when that is.
- One task per block. Decide it the day before.
- Watch for your own signals: rereading the same paragraph, reaching for your phone, yawning. If they come at 70 minutes, stop at 70.
- Take a 15 to 20-minute break, away from the screen: walk, eat, get outside.
- Stop after three. A fourth block is usually worth less than a good rest.
The deep work timer has a 90/20 setting. If 90 minutes is too much to start with, the 52/17 rule is a good stepping stone.
A 90-minute flight
In Flight, a focus session is a real flight. London to Zurich is 1h 27m and London to Munich 1h 38m: one block each, with your plane landing when you're done.