Introduction - Driver Drowsiness Detection: Systems and Solutions - page 4

1.3 What Causes Drowsiness?

Although alcohol and some medications can independently induce sleepiness, the
primary causes of sleepiness and drowsy driving in people without sleep disorders
are sleep restriction, sleep fragmentation and circadian factors.

Sleep Restriction or Loss
Short duration of sleep appears to have the greatest negative effects on alert-
ness [ 11 ]. Although the need for sleep varies among individuals, sleeping 8 h
per 24-h period is common, and 7-9 h is needed to optimize performance.
Experimental evidence shows that sleeping less than 4 consolidated hours per
night impairs performance on vigilance tasks [ 8 ]. Acute sleep loss, even the loss
of one night of sleep, results in extreme sleepiness. The effects of sleep loss are
cumulative [ 2 ]. Regularly losing 1-2 h of sleep a night can create a “sleep debt”
and lead to chronic sleepiness over time. The only way to reduce sleep debt is
to get some sleep. Both external and internal factors can lead to a restriction in
the time available for sleep. External factors include work hours, job and family
responsibilities, and school bus or school opening times. Internal or personal
factors sometimes are involuntary, such as a medication effect that interrupts
sleep. Often, however, reasons for sleep restriction represent a lifestyle choice,
such as the decision to sleep less in order to have more time to work, study,
socialize, or engage in other activities.
Sleep Fragmentation
Sleep is an active process, and adequate time in bed does not mean that
adequate sleep has been obtained. Sleep disruption and fragmentation cause inad-
equate sleep and can negatively affect functioning [ 3 ]. Similar to sleep restriction,
sleep fragmentation can have internal and external causes. The primary internal
cause is illness, including untreated sleep disorders. Externally, disturbances such
as noise, children, activity and lights, a restless spouse, or job-related duties
(e.g., workers who are on call) can interrupt and reduce the quality and quantity
of sleep. Studies of commercial vehicle drivers present similar findings. For
example, the National Transportation Safety Board (NTSB) [ 7 ] concluded that
the critical factors in predicting crashes related to sleepiness were: the duration
of the most recent sleep period, the amount of sleep in the previous 24 h, and
fragmented sleep patterns.
Circadian Factors
As noted earlier, the circadian pacemaker regularly produces feelings of sleepi-
ness during the afternoon and evening, even among people who are not sleep
deprived [ 3 ]. Shift work also can disturb sleep by interfering with circadian
sleep patterns.