The transition from summer time to winter time appears to be associated with a higher crash risk. This is not related to fatigue due to one hour less sleep, but rather to the sudden transition from an evening rush hour in the light to an evening rush hour in the dark, crash risk being higher in the dark . This increased risk remains apparent for several months after winter time has started. Again, this suggests that the higher risk is not so much related to the transition to winter time, but rather to an evening rush hour in the dark. See also SWOV fact sheet Fatigue.