Free 90 Minute Timer — Ultradian Rhythm Focus Block

Ninety minutes aligns with your body’s natural ultradian rhythm — the biological cycle of high and low alertness that repeats roughly every 90 minutes throughout the day. This timer gives you one full cycle of peak cognitive performance before the natural dip arrives.

The Science of 90-Minute Work Blocks

Sleep researcher Nathaniel Kleitman discovered that the same 90-minute cycles governing sleep stages (REM and non-REM alternation) continue during waking hours as ultradian rhythms. During the high phase of each cycle, your brain operates at peak concentration and creative capacity. During the low phase, focus and energy naturally dip. Working in 90-minute blocks followed by 15 to 20-minute breaks aligns your effort with these biological peaks rather than fighting against them.

A 90-minute block suits work that demands deep immersion: academic research and writing, software architecture, strategic planning, creative projects, and complex problem-solving. Shorter blocks interrupt the buildup to flow state. Longer blocks push past the natural attention boundary and produce diminishing returns.

Features

Progress bar that fills across the full 90 minutes, 15 alarm sounds, browser tab title countdown, fullscreen mode, repeat toggle. The timer uses Date.now() calculations to maintain accuracy over the full duration without drift.

Other durations: 60 minute timer, 45 minute timer, 25 minute timer. For the 52/17 method, set a custom 52-minute countdown using the Countdown Timer.