"Sleepiness": What Does It Mean?

By Kerrin Leon White - October 10, 2000

"Sleepiness?" Can there be any confusion about the meaning of such a common word?

In fact, when you try to apply a precise, scientific definition, the most common words make for the worst confusion.

Take one example: "Depression." That is an everyday word, and an almost universal human experience, at least when it refers to a low mood. But in medical diagnostic terms, depression refers to not merely a low mood, but the coexistence of certain symptoms persisting over a certain period of time, representing a highly prevalent, but by no means universal, mental disorder.

When we talk about "sleepiness" in a medical context, as a symptom of a possible disorder or disease, we need a more precise definition than casual conversation requires. As usual, when we commence close inspection of a superficially simple issue, it becomes bewilderingly complicated.

Before we venture further into the jungle of medical jargon, let's back up and look at the "lay" definitions of sleepiness.

Did you notice with alarm that suspicious plural form in which I referred to "lay definitions"?

Commendably, my "dictionary for ordinary people," The Newbury House Dictionary of American English (1996), defines "sleepy" in one way, with only a few words: "needing sleep, tired, (syn.) drowsy." Let us put aside for the moment the fact, evident on reflection, that the three terms used are not necessarily equivalent.

Let's proceed to the opposite extreme, my "dictionary for the excessively erudite," the Oxford English Dictionary, which in my "compact" version occupies merely two volumes, comprising 4,116 pages, legible only with a magnifying glass!

This vast tome doesn't really bear comparison with the first-mentioned dictionary. Its definitions of "sleepy" cite quotations illustrating the word's use in works from each of seven consecutive centuries (14th through 19th)! I will not burden you with these, but only with the summary phrases describing each main category of meaning.

Meanings attributed to the word "sleepy" by the Oxford English Dictionary include the following:

1a. Inclined to sleep, having a difficulty [sic] in keeping awake; drowsy, somnolent. These terms--I hope you will agree--seem more of a kind than those used by the Newbury Dictionary.

1b. Given to sleep; lethargic, heavy. This seems only a faint variation on 1a.

2. Characterized by, appropriate or belonging to, suggestive of, sleep or repose. Lest you think this only more of the same minute differentiation, consider the phrase offered by the dictionary: "a little sleepy trade in salt." This is, of course, a metaphorical use of the word.

3. Inducing sleep; soporific. The dictionary notes this usage is now rare, though it merges with the usage in 2, such as "I will sing you some low, sleepy tune."

So where does this get us? Sleepiness--the state of being sleepy--is mostly defined by the tendency to fall asleep, or difficulty in staying awake. No surprises here!

My American Heritage edition of Stedman's Medical Dictionary (1995) does not dignify the word "sleepiness" with an entry, but the Glossary provided by The International Classification of Sleep Disorders offers a definition which touches upon an important distinction:

"Sleepiness, (Somnolence, Drowsiness): Difficulty in maintaining alert wakefulness so that the person falls asleep if not actively kept aroused. This is not simply a feeling of physical tiredness or listlessness."

Do you see the crucial issue? Sleepiness, when used as a medical term, means in a subjective sense something other than is meant by words often used as synonyms, like fatigue, tiredness, exhaustion, lethargy, torpor, lassitude.

Equally important--in an objective, behavioral sense--sleepiness cannot be measured simply by the tendency to fall asleep or the likelihood of falling asleep, since this may be offset by being "actively kept aroused" if only by internal efforts.

These two points lead us to two major areas of misunderstanding when we refer to sleepiness.

The first issue is common to the words we use to describe subjective experiences. As children first acquiring language, we can learn from an adult what the word "blue" means, by reference to a color both can observe. However, the adult can teach the child the meaning of words for subjective experience only by reference to the adult's own state of mind and body--which the child can appreciate only by external cues such as yawning--or by reference to the child's apparent state, inferred from such behavioral clues. This allows for more opportunity to vary understanding by the two individuals as the what the word "sleepy" means.

In one common example, a person with depression may often describe himself as "sleepy" because, due to fatigue, lack of interest, apathy, etc., he spends a lot of the day in bed, occasionally dropping off to sleep. However, one recent study showed that such depressed people do not in fact sleep more, or fall asleep more easily, than normal people--quite to the contrary! This distinction, if carefully made, helps distinguish the "hypersomnic" depressed person from the person with a sleep disorder causing real excessive sleepiness and excessive sleeping. This is crucial in avoiding the stigmatizing, even nihilistic, effect of applying a psychiatric diagnosis such as depression to a person with a disease of known physical cause such as sleep apnea.

The second point raised by The International Classification of Sleep Disorders Glossary definition of sleepiness has to do with designing and selecting ways of "objectively" measuring sleepiness.

Consider my own favorite example of how an individual may be extraordinarily sleepy yet have a very low likelihood of falling asleep: a soldier on sentry duty. I like this example because it illustrates the importance of exact wording in reference to sleepiness.

If the soldier, visited by his commanding officer, says "I'm feeling very sleepy," he will probably draw words of encouragement to stay awake. If, on the other hand, he says, "I am tending to fall asleep," he may end up in the stockade!

The Epworth Sleepiness Scale has often shown conspicuous limitations to its validity as a measure of sleepiness, generally represented by only modest correlation with other purported measures of sleepiness, or changes in clinical state due to sleep deprivation or treatment for disorders of hypersomnolence.

What about the two main truly "objective" measures of sleepiness--the Multiple Sleep Latency Test (MSLT) and the Maintenance of Wakefulness Test (MWT)? Both employ sleep polysomnography to measure the time delay from "lights out" to first occurrence of sleep during the day.

The difference between the two lies in whether the patient is encouraged by instructions to nap or to stay awake. Thus, they carry at least an aura of scientific technology. The MSLT enjoys extensive clinical use in the diagnosis of narcolepsy, where it also affords the repeated opportunity to document the occurrence of Sleep Onset Rapid Eye Movement Periods, another characteristic of narcolepsy. In the MSLT, people with narcolepsy show the most striking shortening of sleep latency; people with sleep apnea, on the other hand, show a more modest shortening, compared to normals. The much less common MWT may actually have advantages for them.

Both the MSLT and the MWT, however, suffer from the drawback that they represent only imperfect approximations to the real situations encountered by the sleepy person--the situations that the Epworth Scale attempts to access via subjective estimations.

Few people habitually retire several times during the day to a darkened bedroom for 20-30 minutes, to see whether they can either fall asleep or stay awake!

Much more to the point--but also much more difficult--would be ambulatory recording of sleep/wake indicators during a person's "usual" day. This presents fewer technical problems of monitoring than it does practical problems of having the person attempt to live a "normal" day with a conspicuous device attached! Moreover, it presents the formidable difficulty of multiple, non-standardized situations to be compared against some "norms."

Nevertheless, the difficulties presented by recording sleep and waking in a person's normal life are not insuperable. One avenue that has been pursued is the EEG monitoring of the person in one specific, important life activity, such as while driving a truck. Another avenue is the use of less cumbersome, intrusive, and expensive recording devices, such as the Activity Monitor, worn on the wrist, which has shown surprisingly good correlations of generalized body movements with alertness. Yet another, low-tech test of sleepiness is the Eyeblink Test, though this would require repeated observation by another person. Finally, the identification of "standard" situations presenting many common features across numbers of individuals--such as sitting alone in front of a TV versus eating dinner in the presence of others--may be less problematic than supposed.

A favorite caution of one scientific mentor of mine, that I will never forget, is to beware of "looking where it's light instead of where it's lost." That is to say, scientists must avoid the temptation of making research too easy on themselves, at the expense of making it less relevant to the real life problems they attempt to solve!

I trust that, in time, the hardier--or more foolhardy--of medical researchers will come face to face with the real problem of establishing a reliable, valid definition of sleepiness which will unite both its subjective and objective features. Until then, we must all take care what we mean when we tell ourselves--and especially our doctors--that we suffer from excessive "sleepiness"!

For more information on Sleep Apnea, please see our Sleep Apnea Section.