M.Torabi Nami MD, PhDc
Department of Neuroscience
Cognitive Science Studies (ICSS), Tehran 15948 Iran
Sleepiness, tiredness and
fatigue are complaints which must be thoroughly analyzed to
eliminate blur and ambiguity.
Physiological sleepiness (“sleep
pressure”) increases while being awake and additionally underlies
the circadian rhythm with a lower threshold to fall asleep during
Excessive daytime sleepiness (EDS) is considered
normal only after sleep deprivation. Clinically, EDS manifests by
frequents daytime napping and/or reduced alertness with automatic
behavior or - in its extreme form - in recurrent attacks of sudden,
uncontrollable compulsion to sleep also in inappropriate situations
( “sleep attacks”).
EDS is “objectively” addressed by
measuring the mean sleep latency to four to five nap opportunities
throughout the day using the multiple sleep latency test (MSLT) or
the maintenance of wakefulness test (MWT).
EDS denotes both, a
ready entrance into sleep as well as difficulty in staying awake
during daytime or accordingly in inappropriate situations. These
two partially independent aspects of EDS are separately assessed by
the “passive” MSLT and the “active” MWT respectively.
that reason the MSLT and MWT only weakly correlate with each other
when tested over a broad range of patients with EDS. It is important
to keep in mind, that these tests are importantly influenced by a
great variety of factors such as mood, anxiety, and motivation.
“Vigilance” comprises wakefulness, alertness and attention and
therefore is more than just the reciprocal to sleepiness. Cognitive
performance tasks such as Steer Clear Reaction Time Test (SCRTT) or
driving simulators require the complete integrity of vigilance to
achieve normal results. Hypersomnia is usually broadly defined as
the combination of abnormally prolonged night-time sleep (regularly
10 h) with EDS during ≥1 months.
On the other hand, the term
hypersomnia has also been used in a narrower scene for the isolated
abnormality of a prolonged night-time sleep need (10 h).
“Tiredness”, also in colloquial language often used for
sleepiness, in a broader sense also describes the feeling of lack of
energy, motivation and initiative.
These patients seek rest
rather than sleep. They often cannot fall asleep when given the
opportunity in spite of feeling tired, and hence, in an MSLT, do not
show an abnormally short sleep latency. Furthermore, tiredness (and
fatigue) as opposed to sleepiness has a mental (“central”) and
physiological (bodily or “peripheral”) component, which the
patients can readily distinguish. Patients with insomnia, mild sleep
apnea syndrome, or depression rather suffer from mental tiredness
than sleepiness during the day.
The simple subjective
self-assessment using the Epworth Sleepiness Scale (ESS) quite
reliably differentiates between sleepiness and mental tiredness
(without sleepiness), which makes it a widely used test. The term
“fatigue” is also heterogeneously used.
In physiology the
“fatigue” implied a “time on task performance decrement” to
describe decreasing muscle force during a sustained physical effort.
In clinical medicine one distinguishes physical (“peripheral”)
from mental (“central”) fatigue and the term usually denotes a
chronic and more abnormal situation than tiredness.
In a broad
sense “fatigue” implies a deficiency in coping satisfactorily
with mental and physical work load. The chronic fatigue syndrome
entails both mental as well as a physical fatigue (so called
“leaden paralysis” of limbs). Depressive states are often
associated with insomnia and fatigue, but there are also cases with
hypersomnia rather than insomnia ( non organic hypersomnia ,
“atypical depression” or “hypersom