Feeling drowsy did not predict how badly the drivers actually drove
Forty drivers took a simulator test an hour after a daytime dose. How drowsy they said they felt bore no relation to how well they tracked the car ahead.
What the study found
Drivers could not tell from feeling drowsy that they were impaired
Self-rated drowsiness is the check most people run before deciding they are fine to drive. In the same simulator trial, how drowsy the forty drivers said they felt did not predict how badly they tracked the car ahead. The measure people trust and the measure that mattered came apart.
The caveat · One hour-long simulator test
This comes from the same forty drivers and the same four sessions, so it inherits that setting: a simulator, a daytime dose, driving one hour later. A failure of self-rating to predict performance across one hour-long test is not proof that people can never judge their own impairment.