The ability to fall asleep follows a fairly predictable daily pattern. It is easy, for most of us, to fall asleep at night and moderately easy to fall asleep in the middle of the afternoon . This daily pattern is a biological rhythm, known as a circadian rhythm and it is impossible to fight. If you are a day worker, you will feel fatigued during the mid-afternoon and you will feel increasingly fatigued as night time approaches. Aside from artificially fighting the fatigue with caffeine or staying physically active, you can’t avoid these high fatigue times.
The mid-afternoon fatigue is called the post-lunch dip and the night time fatigue is called the 6 hour circadian trough because it is normally between 23:00 and 05:00 hrs. Motor vehicle accidents happen a lot during these two fatigue periods . In fact, single vehicle truck accidents are 3.8 times more likely to occur at the tail end of the 6 hour circadian trough, between 03:00 and 05:00 hrs., than between 08:00 and 16:00 hrs. .
For drivers, this means you should avoid driving during your 6 hour circadian trough and consider taking a siesta during the post-lunch dip instead of driving.
For accident investigators, this means that if you are looking into an accident that occurred in the middle of the afternoon and especially in the middle of the night, look into the role of fatigue in the accident….and remember that although the driver may not have fallen asleep, fatigue could have impaired their overall driving ability and caused the accident.
 Dinges, D. F. (1989). The influence of the human circadian timekeeping system on sleep. In M. H. Kryger, T. Roth, & W. C. Dement (Eds.), Principles and Practice of Sleep Medicine (pp. 153-162). Philadelphia: W. B. Saunders Company.
 See for examples:
Mitler, M., Carskadon, M., Czeiler, C., Dement, W., Dinges, D., & Graeber, R. (1988). Catastrophes, sleep, and public policy: Consensus report. Sleep, 11(1), 100-109.
Folkard, S. (1997). Black times: temporal determinants of transport safety. Accident Analysis and Prevention, 29, 417-430.
 Kecklund, G. & Åkerstedt, T. (1995). Time of day and Swedish road accidents. Shiftwork International Newsletter, 12(1), 31.