The alertness test NASA flies with
For three minutes, watch the box below. At random moments a millisecond counter will appear. Tap (or press space) the instant you see it; the counter stops and shows your reaction time.
- Respond as fast as you can, but don't tap early: false starts count against you.
- The gaps between counters vary unpredictably from 2 to 5 seconds. That's the point.
- Stay with it for the full three minutes; vigilance is what's being measured.
The science
The Psychomotor Vigilance Test is the most widely used objective measure of alertness in sleep science. Invented by David Dinges and John Powell in 1985, it is brutally simple (respond to a stimulus that appears at unpredictable intervals) and that simplicity is its strength: there's nothing to learn, so scores don't improve much with practice, making it ideal for tracking one person across days, shifts or time zones.
Reaction times on the PVT are exquisitely sensitive to sleep loss. Sustained wakefulness and chronic sleep restriction reliably slow responses and, more diagnostically, produce lapses: trials where attention briefly disconnects and the response takes far longer than normal. Lapse counts climb steeply with sleep debt even when people insist they feel fine (Basner & Dinges, 2011). The three-minute PVT-B used here is a validated short form developed for exactly this kind of quick field testing, and versions of it fly on the International Space Station to monitor astronaut fatigue.
Honest caveats
Browser timing on a phone or laptop adds latency (touchscreens typically add 30–80 ms versus laboratory response boxes), so compare your scores against your own baseline, not lab norms. Alert adults on lab hardware typically show mean reaction times around 250 ms; add your device's overhead to that mentally. The interesting experiment is repeated measurement: try it well-rested, sleep-deprived, pre- and post-coffee.
- Dinges, D. F., & Powell, J. W. (1985). Microcomputer analyses of performance on a portable, simple visual RT task during sustained operations. Behavior Research Methods, Instruments, & Computers, 17, 652–655.
- Basner, M., & Dinges, D. F. (2011). Maximizing sensitivity of the Psychomotor Vigilance Test (PVT) to sleep loss. Sleep, 34(5), 581–591.
- Basner, M., Mollicone, D., & Dinges, D. F. (2011). Validity and sensitivity of a brief Psychomotor Vigilance Test (PVT-B) to total and partial sleep deprivation. Acta Astronautica, 69, 949–959.