Sleepiness - A critique of its measurement

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The different dimensions of Sleepiness

Horne (1998) argues that there are two modes of sleep, “Core” and “Optional” sleep. Core sleep refers to an essential component, that is; necessary for the maintenance of normal psychological performance; made up largely by delta electroencephalonographic (EEG) activity, or slow wave sleep (SWS), and also about half of our nightly rapid eye movement (REM) sleep; largely concentrated in the first three or four sleep stages (i.e. the initial 4-5 hours of sleep). Optional sleep basically constitutes the remainder of sleep and is appears to be more dispensable. It is extremely important to note that Horne makes this division on the assumption that it is for the brain and behaviour (in particular the cerebrum), that sleep appears to be the most vital and the only organ which shows signs of restitution. Circumstantial evidence to support this notion is provided by; 1) pointing to the fact that the brain is unable to relax outside of sleep - electrical activity during relaxed wakefulness indicates that the cerebrum continues to work at a high rate; and 2) sleep deprivation studies have pointed to a more profound impairment to cerebral function than to any other part of the brain or body, where functions generally ‘slow down’ more than they do become impaired (e.g. Horne, 1978, 1988; Johnson, 1982).

Accordingly, sleepiness can be subdivided into Core and Optional components. For a ‘normal or healthy sleeper’ optional sleepiness would mostly prevail when the individual sleeps for less time than their usual quota, but still enough to satisfy their core sleep requirement (e.g. 6 hours for an 8-hour sleeper). If however this core sleep requirement is not satisfied, then core sleepiness ensues, as manifested by behavioural and performance decrements. Let us now look at the different tools for measuring sleepiness.

Overview: methods for measuring Sleepiness

There are numerous methods for investigating sleepiness available to the sleep researcher. A brief overview will be provided here with a view to introducing the most commonly used methods and providing a simple description of the key features of each. These methods can generally be sub-divided into ‘Subjective’ methods ‘Objective’ methods. However, it should be noted that the author wishes to pay particular attention to the multiple sleep latency test (MSLT). He proposes that the MSLT provides a particularly good basis for understanding sleepiness and its measurement, but moreover by looking at its implementation in research, will also allow us to look at the interplay between the methods and the methodology; a recurring theme of this essay.

SUBJECTIVE MEASURES

Visual Analogue Scales

This is the simplest test of subjective sleepiness. It is not in itself a recognised measure, but is commonly used in conjunction with the psychomotor vigilance task (PVT- see below). It comprises a 100mm line, with ‘Very Alert’ and ‘Very Sleepy’ at the beginning and end of the line respectively, upon which participants are asked to graphically mark where he/she believes they should be on the scale according to what they are feeling at that moment in time.

Epworth Sleepiness Scale (ESS)

This test was developed by Johns (1991), as a measure of ‘trait’ sleepiness. Thus it measures a person’s general level of daytime sleepiness. A self-administered questionnaire is used to capture the participant’s ratings, on an ordinal scale of 0 (never) to 3 (high chance), the chances of dozing off or falling asleep when in eight different situations that Johns claims are encountered commonly in daily life (e.g. sitting and reading, watching TV etc). The ESS is a measure of ‘trait’ sleepiness (a general value for the participant) and not ‘state’ sleepiness (a more situation- and temporal-specific value). The absence of repeated measures means that the test assumes that the participant’s assessment of their general sleepiness would not vary. Further criticism is levelled at the ESS for failing to refer to the time of day or to the context in which each situation occurs. 

Stanford Sleepiness Scale (SSS)

The SSS was developed by Hoddes and colleagues (1973) and is another state measure, consisting of a scale from 1 to 7 on which participants rate themselves according to how they feel at the time. Some sleep researchers claim that this is the best validated of subjective sleepiness scale (Roehrs, et al., 2000), but the test is not without its criticism. The main criticism of the test is that it is culturally biased due to the language used to describe sleepiness, i.e. while ‘woozy’ and ‘foggy’ might be terms used by Americans, they would not appear to form part of, for example, a British person’s everyday vocabulary. Furthermore, Horne (2003) has recently commented that the semantics of the SSS might account for some of its peculiar findings. Referring to Maclean, et al.’s work (1989), Horne points out that the terms “alert” and “relaxed” tap into different major mood factors on the Profile of Moods States (POMS- Lorr, et al., 1967; McNair, et al., 1967), which might be indicative that the SSS, a ‘gold standard’, might not be a ‘pure’ scale of sleepiness.

Karolinska Sleepiness Scale (KSS)

The KSS is another subjective test, very similar to the SSS. It was validated by Åkerstedt and Gillbert (1990), whose EEG results indicated that during wakefulness, subjective sleepiness is reflected in increased activity in the alpha and theta bands. The main difference to the SSS is that the semantics used in the KSS adhere to just one of the two dimensions that the work of Maclean, et al. indicated the SSS appeared to be measuring; namely the “alertness-sleepiness” dimension. As such the KSS can be viewed as a ‘purer’ scale of sleepiness.

OBJECTIVE MEASURES

EEG

Automatic readings are desirable and these are possible with the EEG; electrodes fixed to the surface of the participant’s scalp pick up the minute electrical activity in the cortex (about a millionth of a volt- Horne, 1988), which can then be amplified by about a million-fold. This information can be transferred to a computer where the sophisticated measurements can be observed in great detail. EEG data is widely accepted as reliable and as just hinted at, its precision is a major advantage. However, experiments with the EEG technology are very expensive and time consuming (particularly as the researcher has to allow time for the participant to adapt to and get properly acquainted with the set-up). Also, although theta activity has typically been seen to indicate sleepiness, there is some confusion over what the best frequency to observe is.

Pupillography

This early method is based on the premise that the pupil diameter and stability decrease with sleepiness. As this measure is seldom used, suffice is to say here that it is not really a valid measure for real-world research.

Vigilance and Reaction Time

There are many variations of the vigilance test that manipulate a basic reaction time task. The Psycho-motor Vigilance Task (PVT) is part of a battery of tests called the NAB (neurobehavioral assessment battery) developed at the Unit for Experimental Psychiatry in the University of Pennsylvania, by Powell and Dinges. It is the most frequently used of its kind (Dinges & Kribbs, 1991). Participants observe a blank box on a computer screen and respond as quickly as possible to the appearance of numbers. These numbers represent a stopwatch that appears as and when it is activated at random intervals. The participant’s reaction time on this task is tested repeatedly over a continuous period of time (usually about 10-15 minutes). The advantages of this simple method lie in its ability to gather reliable and readily workable data easily and efficiently. The assumption made here is that the PVT will detect sleepiness as defined by micro-sleeps, operationalized as a reaction time greater than 0.6 seconds. The minor drawback of this test is that the researcher has to prevent practice effects from causing noise. This is achieved by ensuring that the participant has sufficient practice of the task prior to the test commencing. Although the PVT is widely accepted as a sensitive measure of sleepiness, some question marks remain over the extent to which boredom is a confounding variable.

Maintenance of Wakefulness (MWT)

The MWT measures sleepiness by looking at how long a participant can remain awake (under instructions to try to do so) while sitting or lying down in bed, or sitting down on a chair, in a darkened room. Unfortunately, the MWT has not been standardised, nor has its reliability been established (Roehrs, et al., 2000). The latency to sleep onset is thus the measure and this would appear to have ecological validity; from a clinical perspective the critical issue for patients is their ability to maintain wakefulness. However, Roehrs, et al. suggest that this rationale is probably flawed in that it “assumes that a set of circumstances can be evaluated in the laboratory that will reflect an individual’s probability of staying awake in the real world” (pp. 46). Nonetheless, by removing all the environmental factors that promote wakefulness (i.e. stimulation), the MWT is a more or less useful measure of the maximum likelihood of sleep onset.

Multiple Sleep Latency Test (MSLT)

The MSLT looks at sleep propensity, i.e. it conceptualises sleepiness as the likelihood of falling asleep. It has proved extremely popular with sleep researchers, referred to by Roehrs, et al. (2000) as the “the standard physiological measure of sleepiness” (see Carskadon et al., 1986) – a ‘gold standard’. The test uses standard polysomnographic techniques to measure the latency to fall asleep, on repeated measures at 2-hour intervals during the day. The setting here again is a quiet, dark bedroom. The validity and reliability of this test are considered by some to be well established (see Carskadon, et al., 1986). More recently however the MSLT’s gold standard status has come under considerable scrutiny (Kribbs, et al., 1994; Alexander et al., 1991; Bonnet & Arrand, 1998 & 2001). Johns wrote a report on the ‘Failure of the MSLT as a gold standard’ wherein he claims to provide evidence that the MSLT is less discriminating than both the MWT and the ESS (Johns, 2000).

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Rethinking some Interpretations: Function of Function

The author now wishes to move on from methods to look more closely at some of the conclusions drawn in influential studies and theorise how it is that these results were interpreted in the context of the broader theories of sleep function. It will be proposed that some of these interpretations seem to be lacking in self-criticism and suffering from a monotheistic support of certain methods. This ‘loyalty’ to these methods will be presented as a smoke screen that clouds the researcher’s ability to exercise empiricism.

Looking back at Webb and Agnew’s proposition that most of us are chronically sleep deprived, drawing on the eating and drinking analogy, their interpretation of their results can be seen to be the equivalent of trying to understand eating and drinking behaviour from the singular perspective of physiological need. According to Horne’s Core-Optional hypothesis, only Chore sleepiness can be seen to be largely physiological. The more likely form of sleepiness is likely to consist largely of Optional sleepiness, as this pertains to missing out on some of the latter stages of sleep, which are not essential. This sleepiness is essentially behavioural in nature and can be manipulated as such.

 

Levine, et al. (1988) appeared to support Webb and Agnew’s notion of widespread chronic sleep deprivation. Their results indicated that 40% of the healthy young adults who participated in their study had average sleep onset MSLT scores below ten minutes. Ten minutes is the threshold for diagnosis of EDS. As such the researchers concluded that their participants were likely to be suffering from some sleep deprivation, despite these participants not having complained of this.

An alternative view of these results is offered by Horne (1991), who purports that the low sleep onset scores in Levine, et al.’s study might reflect the participants’ ability to fall asleep through their own volition rather than in response to real sleepiness. The ability to do so might therefore be characteristic of a good sleeper as opposed to someone who has some underlying sleep debt.

Bringing in another example; insomniacs are individuals deemed to be suffering from a ‘lack of sleep’. It would follow that these are people who display increased daytime sleepiness. These might well be people whose sleep is down to a Core level. Stepanski, et al. (1988) reported that the MSLT results for a group of insomniacs indicated that they were just as alert during the day (if not slightly more so) than their age-matched group of normal sleepers were. Consequently, Stepanski and his colleagues believed that these insomniacs might be suffering from “physiological hyperarousal”. A different interpretation of these results is offered by Horne, who suggests that it is more likely that these insomniacs have one way or another reduced their sleep down to a Core level. It is purported that their high level of arousal or general anxiety might well have been an instrumental cause of the elimination of Optional sleep, not the result of sleep loss.

The purpose of detailing the different interpretations of these results is not to take sides with either, but to exemplify that the same set of results can be interpreted in very different ways, inevitably in accordance with the researchers’ more general view of sleep function. This just goes to show that the scientist, if he/she is to be an empiricist, should always maintain a critical eye when looking at reports of research.

Discussion: Looking Ahead

The author has sacrificed indulging in the intricacies of the actual methods in an attempt to get a more holistic view of sleepiness and its measurement by contemplating such issues as; 1) what sleepiness actually is, and 2) how researchers’ interpretations of their findings relate to their view of sleep function. As stated at the start and has hopefully become evident from our exploration of the phenomenon, there appears to be much discordance over what exactly researchers investigating sleepiness are actually measuring. Let us make a first pass at refining our working definition, based on what we have observed thus far. It is apparent that sleepiness is multidimensional… the behavioural components.

The MWT provides an example of this: it has been criticised for lacking ecological validity due to inability to account for/predict all the situational factors that influence his/her ability to stay awake, i.e. stimulation in environment, motivation, circadian rhythm, time of day etc. We are dealing with constructs that are limited in their predictor value… This is not to undermine the usefulness of refining our understanding of the situational factors affecting the relationship between sleepiness and wakefulness. Our understanding of the behavioural dimensions of sleepiness (i.e. of optional sleepiness) is undoubtedly increasing as the emphasis on these in research endeavours continues to increase and progress.

It has been purported that when technological advances permit us to look inside the brain at sufficient detail (i.e. at the cellular level), then perhaps we will be able to get closer to the root of our answer - glial cells might hold the key (Horne, 2001). But until then, there is still much progress to be made in our understanding of the behavioural correlates of this cellular activity that promote or inhibit sleepiness and/or wakefulness…

Driving: alcohol (Horne et al, 2003), counteracting sleepiness with an energy drink (Reyner & Horne, 2002) and early morning sleepiness with caffeine (Reyner & Horne, 2000),  (educating drivers about the dangers of driving when sleepiness detected (Horne, 2002) => conclusion? + also, Horne, 2001- guidelines for road safety policies), driver’s perception of sleepiness- in support of subjective measures (Reyner & Horne, 1998),

Because ultimately we are not endowed with a mechanism to over-ride profound sleepiness, it is of utmost importance that the wider public receive proper education vis-à-vis the putative factors affecting sleepiness and how these can be best counteracted, but more importantly, recognised and acknowledged.

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