Subjective fatigue and subjective sleepiness: two independent
consequences of sleep disorders?
JAMIL L. HOSSAIN, PARVEZ AHMAD, LAWRENCE W. REINISH,
LEONID KAYUMOV, NAHEED K. HOSSAIN and COLIN M. SHAPIRO
Sleep Research Unit and Department of Psychiatry, University of Toronto, Toronto Western Hospital, University Health Network, Toronto,
Accepted in revised form 20 May 2005; received 16 July 2004
The objective of this investigation was to evaluate subjective fatigue versus subjective
sleepiness as independent consequences of sleep disorders. Furthermore, we tried to
explore how these symptoms relate to alertness, depressive symptoms and illness
intrusiveness. In a prospective observational study, 283 sleep-disordered patients
referred to a hospital-based sleep laboratory for various indications over a 1-year
period were evaluated vis-a ` -vis fatigue and sleepiness. All patients completed five
subjective questionnaires, underwent objective sleep recording and attended a clinical
interview with a sleep specialist. The subjective questionnaires included the Epworth
Sleepiness Scale, the Fatigue Severity Scale, the Toronto Hospital Alertness Test, the
Illness Intrusiveness Rating Scale and the Center for Epidemiologic Studies-Depression
Scale. Only 4% of the total sample was referred to the sleep clinic due to a complaint of
excessive fatigue compared with 17% for excessive daytime sleepiness. However, during
the assessment, 64% of referred patients reported pathological fatigue without overlap
of sleepiness and only 4% reported pathological sleepiness without overlap of fatigue.
Pearson’s correlation analysis indicated a weak association (r ¼ 0.18) between
subjective fatigue and sleepiness in the total sample. Analysis of variance testing
showed strong association between those patients with prominent fatigue and
depressive symptoms (P < 0.01) and illness intrusiveness (P < 0.001). The findings
support the notion that subjective fatigue and sleepiness can be independent
manifestations of sleep disorders. Furthermore, predominantly fatigued individuals
with sleep disorders seem vulnerable to additional negative consequences due to
possible interplay between amplified fatigue and psychological distress.
fatigue, sleep disorders, sleepiness, subjective rating scales
The prevalence of fatigue in primary care patients is common
and estimated to occur in 6–45% of patients (Cathebras
et al., 1992; Godwin et al., 1999; Lee et al., 1991; Smets
et al., 1995; Ward et al., 1996). This large range in prevalence
is probably attributable to differences in the working defini-
tion of fatigue, measurement tools and the populations
sampled. In comparison, excessive sleepiness ranges between
5 and 15% in the general population (Johns and Hocking,
1997; Ohayon et al., 1997; Partinen and Hublin, 2000; Roth
and Roehrs, 1996). Although fatigue is a common incidental
finding in a wide range of sleep disorders and has been
documented in clinical reports, it has not been studied as an
independent symptom in the context of sleep disorders. On
the other hand, perceived excessive daytime sleepiness has
gained universal recognition as a primary complaint of sleep
clinic patients and an important public health concern
(Roehrs et al., 2000).
Correspondence: Dr Colin M. Shapiro, Sleep Research Laboratory and
Department of Psychiatry, University of Toronto, Toronto Western
Hospital, University Health Network, ECW-3D, 399 Bathurst Street,
Toronto, Ontario M5T 2S8, Canada. Tel.: +1-416-603-5273; fax:
+1-416-603-5292; e-mail: email@example.com
J. Sleep Res. (2005) 14, 245–253
? 2005 European Sleep Research Society
The terms ?sleepiness? and ?fatigue? are often used inter-
changeably by individuals undergoing sleep deprivation or
sleep disruption (Dinges, 1995). Although fatigue does not
seem to be identical with being sleepy, health care providers
and patients tend to equate sleepiness with fatigue, probably
due to overlap or coexistence of these symptoms in many
physiological and pathological conditions (Shapiro and Kayu-
mov, 2000). For example, sleepiness and fatigue both are
associated with inactivity in many people. Although the two
symptoms may occur together, these symptoms can also be
dissociated. For example, fatigue but not sleepiness may occur
after exercise, during pregnancy and in insomnia patients.
Conversely, sleepiness but not fatigue is a problem in most
narcoleptic patients (Abbey and Shapiro, 1995). Physiologi-
cally, sleepiness seems to be primarily dependent on the length
of prior wakefulness and circadian rhythm status whereas
fatigue depends on the level of exertion (Lichstein et al., 1997).
A cross-association may also occur, i.e. prior wakefulness may
effect fatigue and exertion effort may effect sleepiness. For
example, inactivity or boredom may promote sleepiness and
high exertion may produce increased wakefulness (Torsvall,
While occasional fatigue is a part of the strain of everyday
life, persistent or frequent fatigue is often associated with
conditions of physical or emotional illness and may be
responsive to medical intervention (Belza, 1995; Lee et al.,
1991). While some distinguish physiological fatigue from
pathological fatigue by the inability of the latter to be relieved
by rest, others simply view normal fatigue as being acute and
pathological fatigue as chronic (Carpenito, 1995; Piper, 1989).
Studies in experimental subjects with high fatigue scores
demonstrate a lack of sustained physical functioning and
difficulty meeting household responsibilities, compared with
normal healthy individuals with low fatigue scores (Krupp
et al., 1988; Wessely et al., 1996). However, fatigue continues
to remain a poorly understood problem due to the absence of a
?gold-standard measuring tool? to assess non-specific fatigue
and even fewer ways of managing and treating patients who
complain of fatigue (Shapiro, 1998).
Sleepiness can be defined as being an awake-state in which
an individual has an increased propensity to fall asleep
(Dement, 1993). Excessive sleepiness can be conceptually
defined as the desire or tendency to fall asleep at an
inappropriate time, reflecting the ?ratio? of the total sleep-drive
to the total wake-drive (Hanly and Shapiro, 1995; Johns,
1993). The concept of fatigue is more complex due to a wide
range of interpretations that have been applied to it. Fatigue
can be defined as being primarily a subjective experience that
includes physiological performance decrements and psycholo-
gical impairments such as decreased morale, judgment and
mood (Brown, 1994). However, the physiological and psycho-
logical components have generally not been separately defined.
Fatigue can also be defined with respect to its source as either
work-related factors such as duration, timing, type of work or
non-work-related factors such as general, medical, psychiatric
and sleep disorders.
Although excessive sleepiness is regarded as one of the
cardinal manifestations of sleep disorders and often is accom-
panied by fatigue, many patients with fatigue complain of
insomnia and do not report falling asleep or feeling sleepy at
inappropriate times (Aldrich, 2000). A wide range of sleep
disorders can cause sleep deprivation and sleep disruption and
are known to be contributory factors to excessive fatigue and/
or sleepiness (Chesson et al., 2000; Hanly and Shapiro, 1995;
Lichstein et al., 1997; Ohayon and Shapiro, 2000; Ward et al.,
1996). A few studies have reported a high prevalence of
potentially treatable primary sleep disorders among patients
with chronic high fatigue (Buchwald et al., 1994; Krupp et al.,
1993; Le Bon et al., 2000; Manu et al., 1994). Excessive fatigue
as a daytime consequence has been reported in patients with
chronic insomnia, the most common sleep complaint in
primary health care settings (Chesson et al., 2000; Lichstein
et al., 1997). In contrast, patients with chronic insomnia
typically score in the normal range on subjective measures of
daytime sleepiness (Johns, 1991; Lichstein et al., 1994; Olson
et al., 1998).
A limited number of studies have investigated the relation-
ship between sleepiness and fatigue in sleep-disordered patients
such as obstructive sleep apnea (Aguillard et al., 1998;
Chervin, 2000; Lichstein et al., 1997). These studies have
generally demonstrated a poor agreement between the meas-
ures of fatigue and sleepiness and thus suggest that the two
symptoms may have partial overlap but they are not identical
phenomena and therefore require independent assessment. In
our previous study, we have found a weak correlation between
subjective fatigue and sleepiness rating in 195 shift-workers
and also reported a significantly increased rate of primary
sleep disorders (e.g. sleep apnea, periodic leg movements) in
the subgroup of most-fatigued subjects when compared with
least-fatigued shift-workers (Hossain et al., 2003).
Distinguishing between sleepiness and fatigue is important
because etiology and treatment of these two states may differ.
The relative dichotomy between sleepiness and fatigue and the
under-emphasis of fatigue as an independent consequence of
sleep disorders by many health professionals may have led to
some patients with very treatable sleep disorders not being
referred for sleep assessments. This may result in an under-
diagnosis of sleep disorders, especially when fatigue rather
than sleepiness is emphasized as the chief complaint to the
primary care physicians. This observation may explain the
high prevalence of undiagnosed sleep disorders among
the middle-age population without obvious barriers to health
care (Young et al., 1993, 1997, 2002). For example, it is
estimated that among obstructive sleep apnea sufferers, as
many as 93% of women and 82% of men with the disorder are
undiagnosed (Young et al., 1997).
The primary objective of this investigation was to explore
subjective fatigue versus sleepiness as independent manifesta-
tions of sleep disorders in a clinical population. The secondary
objective was to assess how sleepiness and fatigue relate to
alertness, depressive symptoms and illness intrusiveness in
patients with sleep disorders. An exploratory objective was to
246J. L. Hossain et al.
? 2005 European Sleep Research Society, J. Sleep Res., 14, 245–253
consider hypothetical constructs for pathogenesis and mech-
anism of fatigue and sleepiness in sleep-disordered patients in
view of the findings of this study.
This was a prospective observational study conducted in a
group of sleep-disordered patients who were referred to the
Sleep Clinic at the Toronto Western Hospital for various
indications over a 1-year period. Ethics approval was obtained
from the University Health Network Research Ethics Board
before commencing the study. A total of 283 patients were
selected from a pool of 1200 patients who were screened by the
following inclusion and exclusion criteria:
The inclusion criteria: More than 14 years of age; referred by
a community physician and had a specific indication for sleep
study; completed five subjective questionnaires at the sleep
laboratory; underwent polysomnography; and attended a
clinical interview with a sleep specialist.
The exclusion criteria: Any prior diagnoses of medical
disorders that may cause incremental fatigue or sleepiness; any
medication or substances that would have influenced subject-
ive fatigue or sleepiness; and subjects who did not meet the
Assessment and diagnoses of different sleep disorders was
based on the International Classification of Sleep Disorders
(ICSD) [American Sleep Disorders Association (ASDA),
1997], formulated from the clinical history provided by the
patients in a consultation with a sleep specialist, sleep diary
and detailed questionnaire and polysomnography. The sub-
jects also completed the following questionnaires: the Epworth
Sleepiness Scale (ESS), the Fatigue Severity Scale (FSS), the
Toronto Hospital Alertness Test (THAT), the Illness Intrusive-
ness Rating Scale (IIRS) and the Center for Epidemiologic
Studies-Depression (CES-D) Scale.
The ESS is a self-report questionnaire, which assesses
subjective sleepiness and relies on dozing behavior in eight
different situations of varying soporific nature (Johns, 1991).
The questionnaire asks the respondent to rate the likelihood of
falling asleep on a scale from 0 to 3, were ?0? indicates no chance
and ?3? represents a great chance of dozing. Total ESS score is
thesum ofallthe responsesandranges from 0to24, withhigher
scores reflecting greater sleep propensity. Consistent with the
reports of a number of previous investigations, we adopted a
score of 10 as the cut-off point for normal, above which implied
pathological sleepiness (Chervin et al., 1997; Furuta et al.,
1999; Johns, 1991, 1994; Olson et al., 1998; Sauter et al., 2000).
The FSS (Krupp et al., 1989) is a nine-item self-report
questionnaire that assesses the degree of fatigue intensity on
various functional and behavioral aspects of life and provides
a subjective measurement of daytime fatigue that is largely
independent of daytime sleepiness and depression. Each item is
rated from 1 (strongly disagree) to 7 (strongly agree). Scoring
of the FSS involves calculating the mean score for all
statements. The range of possible scores is 1–7, with higher
scores reflecting greater fatigue. The FSS provides an adequate
means of assessing fatigue intensity within a general popula-
tion, has high internal consistency, strong validation data and
clearly distinguishes between patients and controls (Taylor
et al., 2000). Consistent with the reports of a number of
previous investigations, we adopted 3 being the cut-off point
for normal, above which implied pathological fatigue (Aguil-
lard et al., 1998; Herlofson and Larsen, 2002; Hossain et al.,
2003; Krupp et al., 1989, 1993; Lichstein et al., 1997; Schwartz
et al., 1993).
The THAT (C. M. Shapiro, C. Auch, M. Reimer, L. Kayu-
mov, R. Heslegrave, N. Huterer, H. Driver, G. M. Devins,
unpublished data) is a new approach to the construct of
subjective alertness that assesses a range of activities such as:
ability to concentrate; think of new ideas or focusing on the
task on hand. THAT is a 10-item self-report questionnaire
designed to measure perceived alertness in the preceding week.
The respondent rates 10 different features of alertness on a
scale of 0 (not at all) to 5 (all the time I was awake). Two items
are worded negatively in order to disrupt the tendency toward
a response set and need to be inverted during scoring. Possible
score ranges from 0 to 50 and higher scores indicate a higher
level of alertness and functioning. The interpretation and
clinical utility of the scale at present has limitations due to
unavailable validity and reliability information.
The IIRS is a self-report instrument, measuring the extent to
which one’s illness and/or its treatment ?interfere? with 13 life
domains, important to one’s quality of life (Devins et al.,
1983). Each item is rated from 1 (not very much) to 7 (very
much). The range of possible scores is 13–91 and higher scores
reflect greater illness intrusion. Previous studies of different
populations, which utilized the IIRS have reported it to be
meaningful, reliable and valid measure of the extent of a given
illness interfering with important life domains (Antony et al.,
1998; Devins et al., 1983, 2001).
The CES-D is a 20-item self-report, four-point Likert scale
(response range 0–3) developed to screen for cognitive/affective
symptoms of depression in primary health care settings
(Radloff, 1977). The components include depressed mood,
feelings of guilt, helplessness, worthlessness, loss of appetite,
psychomotor retardation and sleep disturbance. Four items are
worded in a positive direction in order to disrupt the tendency
towards a response set. The total scores can range from 0 to 60
and a score of greater than 16 indicates a clinically significant
level of depressive symptoms (Radloff, 1977; Unutzer et al.,
2002). The CES-D has been used in sleep-disordered popula-
tions and has been found to be a useful symptom scale to
measure depression (Bardwell et al., 1999, 2000, 2003).
SELECTION CRITERIA FOR POSSIBLE
OVERLAP OF FATIGUE AND SLEEPINESS
Association or dissociation of subjective fatigue and sleepiness
was determined by the following criteria: (a) literature derived
cut-off points for normal and pathological scores of the ESS
and FSS rating scales; (b) mean subjective scores of sleepiness
and fatigue in sleep-disordered patients of this sample; and (c)
Fatigue versus sleepiness in sleep disorders247
? 2005 European Sleep Research Society, J. Sleep Res., 14, 245–253
various combinations of normal (low) and pathological (high)
scores of sleepiness and fatigue in this sample.
The data were analyzed by using the Statistical Package for
Social Sciences (SPSS) 11.5 version. One-way analysis of
variance (anova) was utilized to detect the significance of
different subjective symptom scores among the four unique
groups of overlap between fatigue and sleepiness, various
diagnostic categories and between males and females. Tukey
post-hoc multiple comparisons were carried out to determine
which mean scores differed among the categories and the
possibility of type-1 errors was minimized by Bonferroni’s
correction. The relationship between all the subjective meas-
ures from the total subject pool was examined by Pearson’s
correlation. Chi-square and Fisher’s exact tests were carried
out to differentiate the proportion of primary sleep pathology
in the four unique groups as well as between males and
females. Multiple regression testing was used to predict
relationships between fatigue and sleepiness severity and
various independent sleep variables.
The final sample included 283 patients, consisted of 161 males
(56.9%) and 122 females (43.1%). The sample included 37
symptomatic patients on continuous positive airway pressure
(CPAP) therapy who were referred to be assessed for a
co-morbid disorder producing the symptoms (fatigue and/or
sleepiness), notwithstanding adequate control of obstructive
sleep apnea syndrome. The mean age of the sample was
45.2 ± 15.1 (SD) and the mean body mass index (BMI) was
28.8 ± 8.1 (SD).
According to the selection criteria, we categorized all the
subjects into four distinct groups, as shown in Fig. 1.
Of the total sample, only 11 (4%) of the patients were
primarily referred by their physicians for excessive fatigue
complaint. However, based on the FSS, 181 (64%) of patients
in the study group reported pathological fatigue without
overlap of sleepiness. In contrast, 48 (17%) of the patient were
referred due to an excessive sleepiness complaint, and only 11
(4%) reported pathological sleepiness without overlap of
fatigue. Fig. 1 shows frequency of the four groups of overlap
between fatigue and sleepiness.
anova testing showed significant differences in all the
subjective measures between the four unique groups as
shown in Table 1. Tukey post-hoc multiple comparisons
revealed a significant difference in the illness intrusiveness
score between predominantly fatigued (high fatigue/low
sleepiness) and predominantly sleepy (high sleepiness/low
fatigue) groups (P < 0.01). It also showed a significant
difference in alertness and depression scores between high
fatigue/high sleepiness (pathological overlap) and low fatigue/
low sleepiness (normal overlap) groups (P < 0.01) and
between predominantly fatigued (high fatigue/low sleepiness)
and low fatigue/low sleepiness (normal overlap) groups
(P < 0.01).
Statistical relationships between all the subjective measures
from the total subject pool (n ¼ 283) were examined in several
ways including correlations, plots and frequency tables.
Pearson’s product–moment correlation analysis showed a
non-significant association between continuous variables of
subjective fatigue and sleepiness in this sample (r ¼ 0.18).
Moreover, a negative correlation was found between sleepiness
and alertness (r ¼ )0.22) and between fatigue and alertness
(r ¼ )0.38). To appreciate the strength and significance of the
correlation analyses, the model summary of the measured
variables are provided: subjective fatigue and sleepiness
[F(1,281) ¼ 2.60, P < 0.284]; subjective sleepiness and alert-
ness [F(1,281) ¼ 3.38, P < 0.142]; and subjective fatigue and
alertness [F(1,281) ¼ 5.33, P < 0.036].
Mean FSS = 5.1
Mean ESS = 6.9
Mean FSS = 5.4
Mean ESS = 17.2Mean FSS = 2.1
Mean ESS = 5.4
Mean FSS = 1.9
Mean ESS = 14.5
Highly fatigued and
sleepy (n = 54)
Possible association and dissociation of fatigue and sleepiness
non-sleepy (n = 37)
fatigued (n = 181)
sleepy (n = 11)
Percentage of population
Figure 1. Frequency and percentage of the four groups in the sample
(n ¼ 283).
Table 1 Mean (±SD) symptom scores in the four unique groups
45.0 ± 15.0
28.1 ± 6.7
6.9 ± 3.8
5.1 ± 0.9
21.7 ± 11.0
26.3 ± 8.0
42.1 ± 16.6
43.0 ± 14.0
31.5 ± 11.7
17.2 ± 2.7
5.4 ± 1.0
23.0 ± 11.9
23.12 ± 6.8
49.9 ± 17.3
48.2 ± 16.8
27.6 ± 7.6
5.4 ± 2.8
2.1 ± 0.7
13.2 ± 10.6
32.7 ± 9.5
23.9 ± 10.5
50.2 ± 13.6
30.7 ± 6.7
14.5 ± 2.9
1.9 ± 0.6
19.4 ± 11.2
25.0 ± 12.0
30.4 ± 15.0
F(3,279) ¼ 3.0, P < 0.03
F(3,279) ¼ 138.5, P < 0.0001
F(3,279) ¼ 143.0, P < 0.0001
F(3,279) ¼ 6.86, P < 0.001*
F(3,279) ¼ 10.35, P < 0.001*
F(3,279) ¼ 21.0, P < 0.0001*
BMI, body mass index; ESS, Epworth Sleepiness Scale; FSS, Fatigue Severity Scale; CES-D, Center for Epidemiologic Studies-Depression; THAT,
Toronto Hospital Alertness Test; IIRS, Illness Intrusiveness Rating Scale. *Tukey post-hoc multiple comparison test.
248J. L. Hossain et al.
? 2005 European Sleep Research Society, J. Sleep Res., 14, 245–253
The frequency and percentage of primary sleep pathology
and pathological symptom scores in the four unique groups
are shown in Table 2.
Chi-square test showed that the predominantly fatigued
(high fatigue/low sleepiness) patients had more than twice the
illness intrusiveness score when compared with the predomin-
antly sleepy (high sleepiness/low fatigue) patients (P < 0.001).
The frequency of the different diagnostic categories and their
mean subjective symptom scores are shown in Table 3.
Multiple regression analysis was utilized to predict the
relationships between subjective fatigue and sleepiness severity
(dependent variables) and the following independent variables:
apnea–hypopnea index (AHI), periodic leg movements index
(PLMI), arousal index and oxygen desaturation. anova and
Pearson’s correlation were used to test the significance of the
associations and most tests showed non-significant relation-
ships between the symptoms and possible predictors. We also
considered other potentially confounding factors such as age,
gender and BMI. Factorial anova within the regression
analyses was utilized to measure the strength and significance
of the observed relationships and that showed significant
association between subjective sleepiness severity and BMI
[F(1,281) ¼ 10.92, P < 0.001] as well as subjective fatigue and
gender [F(1,281) ¼ 12.41, P < 0.001].
anova testing detected significant differences between males
and females as shown in Table 4. Female patients were found
to be younger, had significantly higher fatigue, depressive
symptoms and a lower alertness scores. Significantly more
male patients (75.0%) were diagnosed with primary sleep
disorders when compared with females (59%). Based on the
Fisher’s exact test, the difference in proportion of primary
sleep pathology between the two groups was significant
(P < 0.005).
The results of this study showed several new and interesting
findings. Only a small percentage of the patients were referred
Table 2 Frequency of primary sleep
disorders and pathological symptom scores
(n ¼ 181)
(n ¼ 54)
(n ¼ 37)
(n ¼ 11)
9 (82) Primary sleep disorders
Pathological subjective scores
Values in parentheses are in percentage.
Table 3 Mean (±SD) symptom scores in the major diagnostic categories
FSS ScoreESS ScoreTHAT Score CES-D ScoreIIRS Score
Obstructive sleep apnea (n ¼ 93)
Periodic leg movements (n ¼ 33)
Restless legs syndrome (n ¼ 12)
Insomnia (n ¼ 33)
Depression (n ¼ 58)
Narcolepsy (n ¼ 9)
Parasomnia (n ¼ 11)
Delayed sleep phase syndrome(n ¼ 15)
FSS, Fatigue Severity Scale; ESS, Epworth Sleepiness Scale; THAT, Toronto Hospital Alertness Test; CES-D, Center for Epidemiologic Studies-
Depression; IIRS, Illness Intrusiveness Rating Scale.
4.4 ± 1.6
4.6 ± 1.4
5.1 ± 1.0
4.4 ± 1.5
4.9 ± 1.4
4.8 ± 1.2
5.2 ± 1.2
4.6 ± 1.6
9.8 ± 5.6
8.7 ± 5.5
11.9 ± 7.3
7.6 ± 5.2
7.7 ± 4.6
16.2 ± 3.5
9.3 ± 6.2
7.0 ± 5.6
28.6 ± 8.8
27.0 ± 7.8
24.3 ± 6.3
28.0 ± 9.1
24.0 ± 8.5
21.5 ± 6.2
21.5 ± 7.1
25.4 ± 7.8
18.9 ± 12.5
20.6 ± 12.5
18.4 ± 11.7
16.8 ± 8.5
27.0 ± 10.5
15.8 ± 7.3
29.4 ± 5.6
19.0 ± 9.9
39.2 ± 20.8
39.9 ± 15.1
39.1 ± 17.3
39.4 ± 16.8
43.9 ± 17.4
39.4 ± 14.6
44.3 ± 18.2
35.3 ± 16.9
Table 4 Mean (±SD) symptom scores in
males and females
Subjective measures Male (n ¼ 161)
47.2 ± 15.8
28.6 ± 6.3
4.5 ± 1.5
9.4 ± 5.3
18.4 ± 10.9
27.6 ± 8.7
39.6 ± 18.3
Female (n ¼ 122)
42.6 ± 13.7
29.0 ± 10.0
4.9 ± 1.5
8.4 ± 5.8
23.8 ± 11.6
24.9 ± 8.2
42.3 ± 17.0
F(1,281) ¼ 6.45, P < 0.01
F(1,281) ¼ 6.0, P < 0.01
F(1,281) ¼ 15.9, P < 0.001
F(1,281) ¼ 6.8, P < 0.01
BMI, body mass index; ESS, Epworth Sleepiness Scale; FSS, Fatigue Severity Scale; CES-D,
Center for Epidemiologic Studies-Depression; THAT, Toronto Hospital Alertness Test; IIRS,
Illness Intrusiveness Rating Scale.
Fatigue versus sleepiness in sleep disorders 249
? 2005 European Sleep Research Society, J. Sleep Res., 14, 245–253
to the sleep clinic due to excessive fatigue complaint compared
with excessive sleepiness. However, data from self-reported
subjective measures of sleepiness and fatigue showed a very
different patient profile as defined by the four distinct
categories of overlap between fatigue and sleepiness. During
assessment, an overwhelming majority of the sample reported
pathological fatigue without overlap of sleepiness and only a
small number reported pathological sleepiness without overlap
of fatigue. Moreover, correlation analyses indicated only a
weak association between the symptoms of fatigue and
sleepiness. We appreciate that subjective fatigue and sleepiness
may have partial overlap but clearly these are not identical
phenomena. A small number of previous studies also reported
weak association between subjective sleepiness and fatigue in
sleep-disordered patients in general and obstructive sleep
apnea in particular (Aguillard et al., 1998; Chervin, 2000;
Hossain et al., 2003; Lichstein et al., 1997). Our findings
support the hypothesis that excessive sleepiness or fatigue can
be independent consequences of sleep disorders.
Moreover, chi-square test showed a significant positive
association between the patients with pathological fatigue and
depressive symptoms and illness intrusiveness. In contrast,
patients with pathological sleepiness yielded low illness
intrusiveness scores. This suggests that a patient’s illness
experience may be influenced by the possible interplay between
amplified fatigue and psychological distress and these factors
may ultimately determine illness-related quality of life.
We found a closer negative association between subjective
fatigue and alertness than that between subjective sleepiness
and alertness in the total sample. Although limited by the
unavailable validity and reliability data, this closer associ-
ation may suggest that the alertness measure (THAT) may
assess the opposite of fatigue rather than sleepiness. It is
important to compare subjective alertness with sleepiness and
fatigue because these three energy states are often used
interchangeably in clinical medicine and may lead to mis-
diagnoses and inappropriate treatment (Shapiro and Kayu-
We measured the significance of different subjective symp-
tom scores among major diagnostic categories. Subjective
sleepiness scores were very high and alertness scores were low
in narcolepsy, which is consistent with the negative correlation
found between these two measures in the total sample.
Insomnia patients had normal subjective sleepiness but repor-
ted severe fatigue. This finding is in support of a number of
previous studies and again emphasizes the dissociation of
sleepiness and fatigue (Chesson et al., 2000; Johns, 1991;
Lichstein et al., 1994, 1997; Olson et al., 1998).
Our sample showed significant differences in subjective
measures between males and females. Female patients were
found to have significantly higher fatigue, depressive symp-
toms and lower alertness scores despite their younger age and
lower proportion with definitive sleep pathology. It seems
plausible that the discrepancies between females and males
may reflect difference in their perception of fatigue, alertness
Pathogenesis of fatigue and sleepiness in patients with
obstructive sleep apnea syndrome and periodic limb move-
ments disorder was explored. Oxygen desaturation and/or
arousal-associated sleep fragmentation are generally acknow-
ledged the clinical consequences of sleep apnea and implicated
as a cause of fatigue and/or sleepiness. However, the lack of
association between the subjective symptoms and suspected
predictors suggests that increased fatigue or sleepiness severity
may be produced by other factors. This is consistent with
previous observations (Aguillard et al., 1998; Chervin, 2000;
Chervin and Aldrich, 1999; Hossain et al., 2003). We also
explored other confounding factors and found significant
relationships between increased BMI and subjective sleepiness
severity and between female gender and increased subjective
fatigue. These findings suggest that multiple factors are
associated with the subjective experience of fatigue and
The definition, etiology and pathogenesis of fatigue remain
controversial and elusive. Sleep disorders and psychiatric
illnesses are considered important correlates of fatigue and
they interface in numerous ways. Most psychiatric patients
have sleep complaints and many sleep-disordered patients have
psychological complaints. The assessment of sleep and wake-
related disorders affecting the sleep continuity and psychiatric
co-morbidity are important factors in fatigue management
(Bultmann et al., 2002; Ford and Kamerow, 1989). Some
studies have reported that there is an inter-relationship
between the causes and consequences of sleep disturbance
and psychiatric illness in general and depression in particular
(Baran and Richert, 2003; Breslau et al., 1996; Livingston
et al., 1993; Shapiro, 2000). Therefore, the success or failure of
treatment of fatigue or sleepiness can be strongly influenced by
expertise in treating the primary and/or secondary sleep/
psychiatric disorders. A number of studies have shown
evidence of psychiatric co-morbidity as a manifestation of
primary sleep disorders, particularly obstructive sleep apnea
(Bardwell et al., 1999, 2000) and reversal of the symptoms
after appropriate therapy (Means et al., 2003; Millman et al.,
A recent study has demonstrated that excessive fatigue in
obstructive sleep apnea patients may be strongly influenced
by depressive symptoms and not apnea severity (Bardwell
et al., 2003). Indeed, in our sample of 37 symptomatic
patients on CPAP therapy after apnea severity was con-
trolled, 21 had (by self-report) a high fatigue score and eight
of them had co-morbid depression. Therefore, it seems
reasonable to suggest that depressive symptoms may account
for a significant portion of the fatigue severity experienced by
obstructive sleep apnea patients even after therapeutic control
of sleep disruption and hypoxia associated with the primary
The visible discordance between subjective measures of
fatigue and sleepiness in this clinical population discredits the
reliance on sleepiness alone to estimate the validity of
complaints of fatigue. These two symptoms can be distinct
from each other in terms of individual perceptions and
250 J. L. Hossain et al.
? 2005 European Sleep Research Society, J. Sleep Res., 14, 245–253
pathophysiology, and are likely to require independent
assessment in sleep-related and medical disorders. The discus-
sion of relative discrepancy between subjective fatigue and
sleepiness has been acknowledged in recent publications and
valuable insights have been offered (Dement et al., 2003;
Horne, 2003). The possibilities that may account for the
relative incongruence between the subjective reports of sleepi-
ness and fatigue as the consequences of sleep disorders and
non-restorative sleep include: (a) subjective sleepiness due to
neurophysiologic sleep drive may not be recognizable until it is
substantial. In other words, individuals do not describe
themselves as sleepy when the drive to sleep is mild or
moderate. On the other hand, subjective fatigue may be
experienced at the level of suboptimal or premorbid sleep drive
and thus it seems to be a earlier indicator of increased need for
sleep; (b) sleep-disordered individuals may not be able to
identify increased sleepiness because of their chronic habitu-
ation to the symptom; (c) perception of sleepiness can be
sufficiently masked or minimized by physical and mental
conditions of high motivation, excitement and competing
needs (e.g. hunger, thirst). In contrast, subjective fatigue is
relatively unaffected by a stimulating environment; (d) per-
sistent fatigue, even in absence of sleepiness, may be a
manifestation of insufficient sleep or a sleep disorder. Female
patients with sleep apnea are more likely to complain of
fatigue, whereas males report more sleepiness (Redline and
Strohl, 1998); (e) unwillingness of some individuals to describe
the subjective state as being sleepy because of the general
perception that increased need for sleep is a sign of personal
weakness or lack of initiative. On the other hand, the
description of being fatigued or tired seems more acceptable
because it is perceived to be the result of hard work or effort.
Limitations of the present investigation need to be taken
into account: (a) the observational study design provides only
association, not causation of the measured variables; (b)
sample size and selection bias within a sleep laboratory in a
tertiary care hospital may not represent an adequately hetero-
geneous population; (c) no objective measure of sleepiness and
fatigue were conducted to compare with the subjective
symptoms; (d) relative discrepancy between the methodologies
of the ESS (direct measure of sleep propensity by measuring
sleepiness behavior) and the FSS (indirect measure of fatigue
intensity by measuring functional impact of fatigue) may have
influenced the results; (e) sample size of the subgroup of
predominantly sleepy (low fatigue/high sleepiness) was small
and that may have influenced the analyses and interpretation;
(f) clinical outcome data of the various diagnostic categories
were not considered within the scope of analyses and
The findings of this study support the notion that subjective
fatigue and sleepiness can be independent manifestations of
sleep disorders and require independent assessment. Patients
may experience either symptom as being more pronounced and
may be able to only distinguish the most overwhelming
symptom. Future investigations should address the limitations
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