Which factor primarily affects a driver's performance due to drowsiness?

Sleep deprivation is the primary factor that affects a driver's performance due to drowsiness because it directly impacts the brain's ability to function effectively. When a driver has not had enough sleep, their attention, reaction times, decision-making skills, and overall alertness are significantly impaired. This can lead to slower responses to traffic signals or changes in the driving environment, making it more difficult to maintain safe driving practices.

While body weight, emotional state, and caffeine consumption can influence driving performance, they do not have the same direct and critical impact as lack of sleep. For example, emotional state can certainly affect concentration, but it is sleep deprivation that fundamentally reduces cognitive function. Similarly, while caffeine can temporarily alleviate tiredness, it is not a substitute for adequate sleep and can only mask the effects of drowsiness rather than eliminate them. In conclusion, prioritizing rest and ensuring proper sleep is essential for safe driving practices.