Introduction - Driver Drowsiness Detection: Systems and Solutions - page 3

Fig. 1.2 Latency to sleep at 2-h intervals across the 24-h day

period of wakefulness, the more pressure builds for sleep and the more difficult it
is to resist [ 4 ]. The circadian pacemaker is an internal body clock that completes
a cycle approximately every 24 h. Homeostatic factors govern circadian factors to
regulate the timing of sleepiness and wakefulness.
These processes create a predictable pattern of two sleepiness peaks, which
commonly occur about 12 h after the mid-sleep period (during the afternoon for
most people who sleep at night) and before the next consolidated sleep period (most
commonly at night, before bedtime) [ 10 ] (Fig. 1.2 ).
It is also worth noticing that the sleep-wake cycle is intrinsic and inevitable, not
a pattern to which people voluntarily adhere or can decide to ignore. Despite the
tendency of society today to give sleep less priority than other activities, sleepiness
and performance impairment are neurobiological responses of the human brain to
sleep deprivation.
Sleep and wakefulness are influenced by the light/dark cycle, which in humans
most often means wakefulness during daylight and sleep during darkness. People
whose sleep is out of phase with this cycle, such as night workers, air crews,
and travelers who cross several time zones, can experience sleep loss and sleep
disruption that reduce alertness [ 5 , 12 ].
In medical terms, sleep can be divided into three stages: awake, non-rapid
eye movement stage (NREM) and rapid eye movement stage (REM). The sleepy
(drowsy) interval—i.e, transition from awake to asleep—occurs during the NREM
stage [ 1 ].