Keyboard Cadence As A Fatigue Signal: WPM, KPM, And Error Rate
Typing speed is a weak signal in isolation and a strong one across a session. The drop from minute 5 to minute 35 is what tells you something, not the absolute number.
Why typing speed is not the point
A lot of productivity tools that read keyboard activity stop at WPM and treat the number as a verdict on focus. That is the wrong unit of analysis. People type at very different speeds, and even the same person varies by task — a code review block looks nothing like a writing block.
The useful question is not how fast you type. It is how fast you type relative to your own start of the session.
WPM as a session-shape signal
Sarenica records WPM per minute and rolls it up into early, middle, and late thirds of every session. The interesting number is the difference, not the level.
A clean session looks roughly flat across the three thirds, or even climbs slightly. A fatigue-prone session shows a clear decline by the last third. That is the shape your weekly report keys off when it says "30-45 minute sessions look more demanding than 15-30 minute ones".
KPM and burst patterns
KPM (keystrokes per minute) catches things WPM does not — code, shortcuts, navigation, anything that does not produce words. It moves more with task type than with strain, but the burst pattern across a session is a useful tell.
A long-session pattern of irregular KPM bursts followed by long quiet stretches usually means task-switching, which itself correlates with fragmented focus.
Error rate, the cleanest signal
Error rate is the percentage of keystrokes that result in a backspace or correction within a short window. It does not care about your normal speed, your task, or your typing style. It rises with strain and rarely drops back down within the same session.
In practice, error rate is the keyboard signal that most reliably correlates with the eye and posture signals. When all three rise together in the late third of a session, the weekly report has a strong case for "shorter blocks here".
How weekly reports use it
Keyboard signals do not appear as their own section in your weekly progress report. They feed into the productive density and focus drift calculations upstream. That is why a report can call out "your 30-45 minute blocks lose productivity" without ever mentioning typing — the typing pattern is one of the inputs that finding rests on.
If you ever want to see the underlying pattern, the in-app session view shows the early/mid/late thirds for any saved session. That is the right place to verify a weekly finding feels real.