Asleep drivers fail to brake before impact, and therefore tend to crash at higher speeds, hence the greater seriousness of the accident.
However, without an objective measure of tiredness (unlike alcohol which can be measured in blood), company drivers and employers must take responsibility for this issue, according to safety organisation Brake.
It interviewed 500 drivers, of whom 305 (61%) were company car drivers, and only one-third (33%) recognised anti-tiredness measures.
These measures include having enough rest before setting off, taking a break every two hours, drinking caffeine and sleeping if tired. Only 2% of drivers follow Department for Transport advice, which is having a caffeine drink and a 10 minute sleep.
Other commonly taken steps are ineffective in combating tiredness, such as winding down a window or listening to loud music.
One in five motorists drive for more than the recommended two hours without a break, and when they do stop, 80% do so for reasons other than to have a rest, such as refuelling.
Mary Williams chief executive of Brake: 'It is time everyone took responsibility for ensuring they are not tired and the Government took its obligations seriously to ensure it spends the necessary cash on barriers, enforcement of road safety, and education campaigns on TV.'