Sleep deprivation can have a similar effect as drinking alcohol. Being awake for 18 hours straight makes them drive like they have a blood-alcohol level of .05.
Both drowsy driving and drunk driving make it hard to pay attention to the road and negatively impact how fast a trucker can make decisions. A drowsy driver may not be able to break or swerve quickly enough if something happens to be in front of them, which often leads to crashes.