8.2.1 General considerations
Excessive daytime sleepiness, which manifests itself as a tendency to doze at inappropriate times when intending to stay awake, can arise from many causes and is associated with an increased risk of motor vehicle crashes.3–7 It is important to distinguish sleepiness (the tendency to fall asleep) from fatigue or tiredness that is not associated with a tendency to fall asleep. Many chronic illnesses cause fatigue without increased sleepiness.
Increased sleepiness during the daytime in otherwise normal people may be due to prior sleep deprivation (restricting the time for sleep), poor sleep hygiene habits, irregular sleep–wake schedules or the influence of sedative medications, including alcohol. Insufficient sleep (less than five hours) prior to driving is strongly related to motor vehicle crash risk.3 Excessive daytime sleepiness may also result from a number of medical sleep disorders including the sleep apnoea syndromes (obstructive sleep apnoea, central sleep apnoea and nocturnal hypoventilation), periodic limb movement disorder, circadian rhythm disturbances (e.g. advanced or delayed sleep phase syndrome), some forms of insomnia and narcolepsy.