Methodologies for Assessing Sleepiness

There are various methods for assessing an individual's level of sleepiness. These include behavioral indicators, subjective measures, and physiologic measures. Behavioral indicators of sleepiness include yawning, ptosis, decreased social interaction, and microsleep events. Many of these behaviors are difficult to quantify.

Subjective measures include various types of numeric or visual analogue scales that attempt to measure how sleepy the individual feels. These methods are used frequently in studies of sleepiness because they are very easy to implement; however, subjective feelings of fatigue and sleepiness are often underestimated by individuals in comparison with their level of physiologic sleepiness. Both behavioral and subjective sleepiness can be masked by a stimulating environment, even though the individual remains physiologically sleepy. When environmental stimuli wane, physiologic sleepiness manifests itself as an overwhelming need to fall asleep. A person who is physiologically alert does not experience sleepiness as environmental stimuli decreases. For example, without physiologic sleepiness, an individual may become bored during a lecture but does not fall asleep.

Physiologic (objective) measures of sleepiness have been used extensively in sleep research and sleep medicine. Carskadon and Dement and colleagues ^{[363] [364] [365]} developed the Multiple Sleep Latency Test (MSLT), which is now the standard method for quantifying daytime sleepiness. The MSLT tests the propensity of an individual to fall asleep in a sleep-inducing environment during the daytime. A short sleep latency (i.e., falling asleep quickly) is a sign of increased sleepiness whereas a long sleep latency is a sign of decreased sleepiness. A normal MSLT score is greater than 10 minutes, whereas pathologic daytime sleepiness is defined as less than or equal to 5 minutes. Pathologic levels correspond to the daytime sleepiness typically seen in patients with narcolepsy, sleep apnea, or in healthy individuals deprived of sleep for 24 hours.