Ultradian Rhythms: The 90-Minute Focus Cycle

Ultradian rhythms are biological cycles shorter than 24 hours. The most important for productivity is the Basic Rest-Activity Cycle (BRAC) — a roughly 90-minute oscillation between high and low alertness that structures both sleep and waking cognition.

The practical takeaway is simple: work in focused blocks of about 90 minutes, then take a genuine 15–20 minute break. This pattern has been popularized in 2026 by figures like neuroscientist Andrew Huberman, who notes your brain can sustain deep focus for roughly 90 minutes before key neurochemicals begin to drop.

Why 25 minutes can be too short

It takes 10–20 minutes for your brain to reach full engagement on a complex task — the costly "ramp-up" that makes context-switching so damaging. A 25-minute block may give you only 5–15 minutes of peak performance before the timer interrupts. A 90-minute block lets you move through the ramp and into genuine deep focus.

The practical formula

• 90-minute focused block (including a ~10-minute ramp phase at the start).
• Followed by a genuine 15–20 minute break — movement, daylight, water, no screens.
• Repeat for 3–4 cycles per day; most people max out around 4 hours of true deep work.
• Schedule your hardest work in cycles 1 and 2 (morning), lighter work after lunch.
• Avoid focused blocks in the last 90 minutes before bed.

Listen to your body, not just the clock

The 90-minute mark is an average; real cycles range from about 80 to 120 minutes. If you're in genuine flow when the timer rings, extend by 15–20 minutes and then break. But if you consistently push well past 90 minutes without rest, you'll notice diminishing returns, more errors and growing fatigue.

If you're new to long blocks, build up gradually. Start with what feels manageable and extend your focus stamina over a couple of weeks.