FIGURE 1 Venn diagram for introspective, physiological, and manifest sleepiness.
"physiological sleepiness" can be thought of as the underlying biologic drive to sleep indexed by the amount of time it takes to fall asleep, given the opportunity. Finally, "manifest sleepiness" reflects an individual's inability to volitionally remain awake. This state can be indexed by behavioral signs of sleepiness or sleep onset (eye closure, head bobbing, snoring) or by performance deficit on a wide variety of psychomotor and cognitive tasks. Although introspective, physiological, and manifest sleepiness levels may stem from a common source, tests assessing sleepiness in these different realms cannot be used interchangeably. Furthermore, attempts to use these measures interchangeably miss the importance of the differences between them.
At extreme ends of the spectrum, sleepiness measures may be concordant. That is, a male soldier who has remained awake continuously for 48 hours, when asked at 4:00 a.m. if he is sleepy will most likely respond affirmatively. If provided the opportunity to lie down on a comfortable bed in a dark, quiet room he would probably fall asleep rapidly (in 5 minutes, or less). Furthermore, if he sits down in a comfortable chair in a nonstimulating environment (a dark, quiet, and warm room), he may be unable to remain awake (unless provided strong coffee or other stimulants). In such a circumstance, introspective, physiological, and manifest sleepiness measures would all agree. Conversely, a woman on vacation who has caught up on her sleep to the point that she spontaneously awakens in the morning and feels alert all day will typically not fall asleep at 7:00 p.m., even if she lays down on a comfortable bed. Nor does she have any difficulty remaining awake sitting in a comfortable chair in a darkened, quiet environment. Thus, at this other end of the spectrum, there is a convergence in measures indicating full alertness.
It is the state in between full alertness and maximal sleepiness that provide a challenge for understanding test measurement. For example, if a couple has stayed up all night (24 hours) and is watching the sunrise at 7:00 a.m. they may not feel sleepy. However, 2.5 hours ago at 4:30 a.m., they could barely rally enough to stay awake, but that feeling of overwhelming sleepiness has dissipated. Nonetheless, if given the opportunity to lie down in a comfortable bed in a dark and quiet room, they would fall asleep instantly (unless they have been drinking coffee all night or taking stimulants). Depending on their individual ability to maintain alertness, they may be able to stay awake for more than 20 minutes if seated in a dark and quiet room. Thus, the introspective and manifest measures of sleepiness are negative, while the physiological measure is positive. Furthermore, even though they are now awake longer, 2.5 hours ago, introspective and physiological measures were positive but manifest sleepiness level was not. This illustrates the importance of understanding the differences between measurements and not generalizing results from one to another.
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