Your circadian rhythm leaks into your typing. A digital phenotyping framework published in npj Digital Medicine (2025) predicts sleep duration from smartphone typing dynamics — not from what you type, but from how your typing speed, error rate, and inter-keystroke intervals vary across the 24-hour cycle.
The framework uses these patterns to estimate circadian phase, then tracks phase shifts during time zone transitions. When you fly from New York to London, your typing dynamics jet-lag at a measurable rate, and the recovery trajectory maps onto the known ~1 hour/day circadian adjustment speed.
The finding that typing reveals circadian state seems obvious in retrospect — of course motor performance varies with alertness, and alertness varies with circadian phase. But the practical implications are not obvious at all. This means any smartphone can function as a passive circadian monitor without a wearable, without an app the user actively engages with, without any specialized sensor. The data already exists in the keyboard events that every phone already logs.
The subtler insight: the circadian signal in typing dynamics is robust enough to survive the enormous noise of everyday variation in typing context. You type differently when composing an email versus texting a friend versus entering a password. The circadian modulation rides on top of all this task-dependent variation and remains detectable. The signal persists because the underlying motor system — the one generating the keystrokes — is controlled by the circadian clock regardless of what the cognitive system above it is doing.
The phone doesn't know your circadian phase. But it knows your typing rhythm, and the rhythm knows your phase. The measurement is doubly indirect, yet works.