324 Insomnia
Kapoor, 1995; Everson, 1997), evidence for a relationship be-
tween naturalistic sleep loss (i.e., clinical insomnia) on immu-
nity are much more sparse. One sleep laboratory experiment
(Irwin, Smith, & Gillin, 1992) has shown that total duration of
sleep, sleep ef“ciency, and duration of NREM sleep were pos-
itively correlated with natural killer (NK) cell activity, both in
depressed and nondepressed individuals. NK cells are be-
lieved to provide defense against cancer and virus-infected
cells. Cover and Irwin (1994) found that initial insomnia was
one of only two symptoms measured by the Hamilton De-
pression Rating Scale that were signi“cantly associated with
NK cell activity. Similar results were obtained in a sample of
women at risk for cervical cancer (Savard et al., 1999). Higher
sleep satisfaction was associated with a higher concentration
of helper T cells in circulating blood, a T-lymphocyte subset
that carries the CD4 marker; these cells have several functions
(e.g., activate B cells to generate antibodies, activate cytotoxic
T cells) and would be especially relevant for the progression
of cervical cancer. This effect was still present, even after con-
trolling for the variance explained by depression. Collectively,
these studies suggest that insomnia may have an immunosup-
pressive effect. However, the cross-sectional nature of these
data precludes any conclusion about a causal relationship. Ad-
ditional experimental studies are needed to examine more rig-
orously the effect of insomnia on immunity, and the extent to
which immune alterations are clinically signi“cant in terms of
their in”uence on health status.
Overall, given the reciprocal relationship between stress,
somatic/health factors and sleep, as well as the cross-
sectional nature of much of insomnia research, additional
longitudinal investigations are necessary to better understand
the clinical correlates and risk factors of insomnia.
The Impact of Insomnia
Insomnia is associated with signi“cant consequences in one
or more of the following areas: health, quality of life, social
and occupational functioning, economics, and public safety.
The most immediate and direct consequences of insomnia in-
volve daytime fatigue, attention and concentration problems,
reduced motivation, and mood disturbances (irritability, dys-
phoria) (Zammit, Weiner, Damato, Sillup, & McMillan,
1999). While these effects are fairly self-limited when sleep
dif“culties are situational, persistent insomnia can reduce
quality of life, cause emotional distress, and even increase the
risk of major depression (Breslau et al., 1996; Ford &
Kamerow, 1989). There are also signi“cant functional im-
pairments (e.g., work absenteeism and diminished productiv-
ity) that have been linked to insomnia (Simon & VonKorff,
1997). Attention problems and reduced vigilance also can
contribute to accidents on the road or at work. Individuals
with insomnia are more than twice as likely as good sleepers
to report fatigue as having been a factor in their motor vehi-
cle accident (5% versus 2%; Gallup, 1991). More than 50%
of night workers acknowledged having fallen asleep on the
job at least once. Sleepiness has also been implicated in sev-
eral major industrial accidents (e.g., Chernobyl nuclear acci-
dent), all occurring in the middle of the night. Although these
accidents are probably related more to sleep deprivation than
insomnia per se, it does highlight the potential impact of lack
of sleep on public health safety.
The prevalence of insomnia and the apparent chronicity
and morbidity of this condition lead to the important ques-
tion: What are the costs associated with this condition? Direct
costs include the cost of all products used (prescription, over-
the-counter, natural, etc.) and consultations for insomnia
symptoms. Indirect costs of insomnia include those related to
work absenteeism, low productivity, poor job performance,
and accidents. Individuals with insomnia complaints report
greater functional impairments, take more frequent sick
leaves, and utilize health services more frequently (Leigh,
1991; Mellinger et al., 1985; Simon & VonKorff, 1997) than
those without sleep complaints. The total cost for substances
used in the United States in 1995 to treat insomnia was esti-
mated at $1.97 billion (less than half of this was for prescrip-
tion medication), the total of all direct costs being estimated
at $13.9 billion. If indirect and associated (e.g. accidents)
expenses are included, the total cost of insomnia in the
United States is estimated as between $30 and $35 billion
(Walsh & Engelhard, 1999). These “gures are very approxi-
mate because they are usually based on retrospective esti-
mates or on available databases (see Chilcott & Shapiro,
1996; Leger, Levey, & Paillard, 1999). Prospective and lon-
gitudinal studies are needed to measure more accurately the
costs of insomnia.EVALUATION OF SLEEP
COMPLAINTS/DISORDERSIt is a common mistake to view insomnia as a simple sympto-
matic problem for which a simple diagnostic procedure can be
used and an all-purpose intervention applied. Because of the
heterogeneous nature of insomnia, a thorough evaluation of
medical, psychological, pharmacological, and environmental
factors is essential to make an accurate diagnosis and design
an appropriate treatment. Ideally, the evaluation should be
multifocused and include complementary assessment meth-
ods such as a clinical interview, daily self-monitoring of sleep
habits, and self-report measures. A sleep laboratory evalua-