Studies suggest that driver fatigue is a factor in as many as 20% of all accidents and a quarter of all driver deaths[i]. In this first of a two-part article, SCT’s Mario Pierobon focuses on driver fatigue, and specifically how it can be defined, how it originates, its symptoms, and ways to measure it.

Between Awake and Asleep

“People exposed to shift-work can have major disruptions in sleep and circadian rhythms,” say Saroj Lal and Ashley Craig in an academic journal entitled ‘A critical review of the psychophysiology of driver fatigue’[ii]. Indeed, in transportation systems, fatigue represents a serious problem during driving, according to Lal & Craig. “Furthermore, having sleep disorders will more than likely result in higher levels of fatigue and therefore increase risks of accidents. For example, drivers who suffer from hypersomnia have increased risks of being involved in car accidents,” the researchers say.

Fatigue can be described as the transitory period between awake and sleep and, if uninterrupted, it can lead to sleep, according to Lal & Craig. “To better define the term fatigue, it is helpful to note that it can be classified into physical and mental categories. Mental fatigue is believed to be psychological in nature whereas physical fatigue is considered synonymous with muscle fatigue.”

A general feeling of weariness, impaired activity and feelings of inhibition are the major symptom of mental fatigue, observe Lal & Craig. “At any moment a person is in one particular functional state, somewhere between the extremes of sleep and of a state of alarm. When defining fatigue, it is important to discuss the association between fatigue and boredom, since one may affect or instigate the other.”

Origins & Symptoms

Several factors can explain fatigue and drowsy driving. “Different physiological and psychophysiological processes can be linked to fluctuations of activation, arousal, alertness and vigilance,” say Pierre Thieffault and Jacques Bergeron in an academic paper entitled ‘Monotony of road environment and driver fatigue: a simulator study’[iii].

Lal & Craig explain that a stream of impulses from the sensory organs that is combined with feedback to the cerebral cortex stimulate the reticular activating system and maintain the central nervous system (CNS) in a high state of readiness. “When stimuli are few, the stream of sensory impulses diminishes, reducing the level of activation of the cerebrum, thereby raising the chances of inducing a fatigued (or bored) functional state.”


Driver fatigue is more frequent on monotonous road environments such as on motorways, observe Thieffault & Bergeron. “Psychological reactions to monotony consist mainly of feelings of boredom and drowsiness coupled with loss of interest of performing the task at hand,” they affirm. “Stress can also be related to these conditions.” [iv]

Muscular fatigue is a reduced performance of a muscle after stress and is characterised by reduced muscular power and movement, explain Lal & Craig. “There is an emerging recognition that a sleepiness: fatigue state contributes to deterioration in performance, which may lead to errors and increase the risk of accidents. Since fatigue affects attention and performance, it is important to consider ‘vigilance’ with which it overlaps.”

Measuring Fatigue

Difficulties in maintaining skilled performance are associated with any activity if pursued long enough, and this is the case for driving, according to Lal & Craig as “the search for a reliable indicator of fatigue is still elusive even though the technology for measuring these factors have greatly improved”.

Among the indicators to measure the level of alertness there is the electroencephalography (EEG) which is a fatigue indicator, according to Lal & Craig. “The EGG is generated by excitatory and inhibitory postsynaptic potentials of cortical nerve cells,” they say. “Conceptually, ERPs (event-related potentials – editor’s note) are regarded as a manifestation of specific psychological processes.”

Another promising measure of fatigue and drowsiness is eye movement, observe Lal & Craig. “Since there are rich sensory and motor connections between the eye and the brain, eye movement can provide valuable warning signs of drowsiness. Its predictive ability of arousal status is becoming accepted,” they say.



[i] The Royal Society for the Prevention of Accidents “Driver Fatigue and Road Accidents Factsheet” July 2020 (
[ii] Saroj K.L. Lal, Ashley Craig, A critical review of the psychophysiology of driver fatigue, Biological Psychology, Volume 55, Issue 3, 2001, Pages 173-194,
[iii] Pierre Thiffault, Jacques Bergeron, Monotony of road environment and driver fatigue: a simulator study, Accident Analysis & Prevention, Volume 35, Issue 3, 2003, Pages 381-391, (
[iv] Thieffault & Bergeron