The shift worker cannot sleep when sleep is desired, needed, or expected by his own body, and thus, is likely to suffer from chronic sleep loss . Not surprisingly, being engaged in shift work is associated with increased traffic risks. For example, a study by Folkard  showed that the risk of being in a single-vehicle accident at 3 o’clock in the morning was 50% above the baseline after four successive night shifts.
Fatigue-related crashes tend to occur in two distinct periods of the day – between midnight and 6 am, and between about 2 pm and 4 pm . These periods coincide with typical low-points in our daily pattern of alertness, or circadian rhythm.
Drivers who work irregular schedules are most likely to be affected by the body’s natural desire to sleep during the night. Studies on driver fatigue have typically used vehicle control and psychophysiological measures as indices of driver drowsiness. These studies have found that time of day has a larger impact on driver fatigue than time on task .
Shift workers form a large part of the working population. Approximately 24% of the European population work on a regular 8-hour schedule during daylight hours (07.00-18.00) and 5 days per week. 17% are engaged in shift work and 14% have long shifts (at least 10h) on a regular basis .