Disorders and disturbances of sleep are widespread and can have significant consequences for affected individuals as well as economic and other consequences for society. The US National Transportation Safety Board has, according to Dr. Charles Czeisler, member of the Institute of Medicine and Director of the Harvard University Medical School Division of Sleep Medicine at Brigham and Women's Hospital, discovered that the leading cause (31%) of fatal-to-the-driver heavy truck crashes is fatigue related (though rarely associated directly with sleep disorders, such as sleep apnea), with drugs and alcohol as the number two cause (29%). Sleep deprivation has also been a significant factor in dramatic accidents, such as the Exxon Valdez oil spill, the nuclear incidents at Chernobyl and Three Mile Island and the explosion of the space shuttle "Challenger".