The lapse is the signature
When people are tested after restricted sleep, their average reaction time gets worse — but the average understates the damage. What really changes is the appearance of lapses: individual trials that take far longer than the rest, as attention drops out entirely for a moment.
On this test that shows up as a wide spread. Four trials at 250 ms and one at 600 ms is the classic sleep-deprived pattern, and it is far more dangerous in the real world than a uniformly slow set, because the failure arrives without warning.
A tired driver is not consistently slower. They are normal, normal, normal, and then absent for two seconds.
It accumulates quietly
Sleeping an hour or two short of what you need does not feel dramatic, but performance on attention and reaction tasks degrades night after night across a week of restriction, while subjective sleepiness levels off. People adapt to feeling tired long before their performance adapts, which is why self-assessment is so unreliable.
The practical consequence is that the worst nights to trust your own judgement about alertness are the ones after several mediocre nights in a row.
Measure it on yourself
- Test five trials at the same time of day for a week, same device, and write down your sleep for each night.
- Compare the spread across days rather than the average — the spread is the sensitive measure.
- Test once after an unusually short night and note how normal you feel while the number says otherwise.
- Test again after a full recovery night. Most people are surprised by how much of the gap closes in one night.
What helps besides more sleep
Nothing helps nearly as much as more sleep, but consistency of timing comes second: going to bed and getting up at the same time stabilises performance more than an occasional long lie-in does. A short nap of 15 to 20 minutes restores alertness for a few hours without the grogginess of a longer one. Caffeine masks the feeling of impairment while leaving part of it in place, which is useful for a deadline and dangerous behind the wheel.
If your test scores are consistently poor and your sleep is consistently short, the score is telling you about the sleep. That is the finding, and there is no drill on this site that fixes it.