People getting less than 7 hours sleep are more likely to be involved in road collisions.
A new study shows the risk is greatest for drivers who've slept for less than four hours.
Researchers found that drivers getting less than 4 hours sleep were 15 times more likely to be responsible for a car crash, compared to those getting the recommended 7 to 9 hours sleep.
Driver fatigue contibutes to 1 in 5 deaths on Ireland's roads.
Concerns are being raised about the risks associated with "fatigued" delivery drivers ahead of the busy Christmas period, starting with Black Friday.
The Mayo Road Safety Office says exhausted delivery drivers could pose a road safety risk over the coming weeks, as thousands of staff work long hours to cope with the rush of online orders for the festive season.
Mayo Road Safety Officer Noel Gibbons says driving while tired reduces the ability to concentrate and be vigilant.
He says this applies, not just to fleet drivers, but to people who stay up at night shopping on line to get early bargains ahead of Black Friday, and are driving early the next day.