Google was forced to pull the plug on early self-driving car experiments after its test drivers fell asleep or applied makeup at the wheel, the tech giant has revealed.

In early experiments with at its self-driving car division, Google found test drivers in cars equipped to perform advanced driver assistance falling asleep at the wheel while at 55mph on a motorway.

The findings led Google to drop its highway driving assist feature in 2013, instead moving to research safer, fully-automated driverless cars, according to TechCrunch.

The company's autonomous car division Waymo said drivers who were filmed using semi-autonomous cars in on the road testing took to doing makeup, hunting for cables and electronic devices and even sleeping, rather than keeping an eye on the road ahead.

"We shut down this aspect of the project a couple of days after seeing that," said Waymo chief executive John Krafcik.