Fatigue does not just make you feel sleepy. It damages many of the skills a driver needs most. Vision becomes harder to focus. Peripheral awareness narrows. Reaction time slows down, which means you may start braking or steering a fraction of a second too late.
Fatigue also weakens your judgment. A tired driver may decide to run a yellow light that a rested driver would stop for, or may underestimate how fast a gap is closing. Poor judgment and slowed reaction time together are a dangerous combination.
On long drives or boring roads, fatigue can lead to highway hypnosis — a mental state where the driver continues to follow the road mechanically but is not truly alert. A driver in this state may not notice a sudden change in traffic, a stop sign, or a curve until it is too late.
Common effects include poor vision and weak perception, slower reaction time, poor judgment of distance and speed, more risk-taking, and highway hypnosis on long straight roads.