Abstract
Sleepiness during activities impairs brain function and is a cause of reduced motivation, mood, and performance. It is also a major factor in causing human errors. Sleepiness can be categorized into subjective sleepiness, which is the feeling of being sleepy, and physiological sleepiness, which is the actual act of falling asleep. The factors that cause sleepiness during activities include the accumulation of sleep debt resulting from insufficient sleep, poor sleep quality, or sleep disorders. Sleep environment is one of the causes of sleep-loss and poor sleep quality. This article provides an overview of the effects of sleep deprivation on brain function, methods for evaluating subjective and physiological sleepiness, and human errors caused by sleepiness, including traffic accidents
