Driver fatigue is not an uncommon factor in collisions, especially while travelling or on the highway. Granted, fatigue may set in more frequently at night, but daytime drivers may be just as susceptible. Time, distance, and being mentally/physically alert are your friends while driving. Fatigue can greatly reduce your ability to react or interpret road conditions, hazards, and other factors that impact safe operation of your vehicle. Pay attention to warning signs: dozing off, heavy eyelids, yawning, drifting, just to mention a few.
The US Department of Transportation offers some good information and tips to recognize and avoid driver fatigue. The information targets commercial drivers, but is applicable to all drivers.