What leads drivers to illicitly nap during conditionally automated driving?

Automation misuse can cause traffic hazards when drivers over-rely on automation and use it in ways that are  not intended by the designers of the system. Automation abuse refers to designers of automation designing systems  without regard to the consequences for human performance. In a driving simulator study, half of the participants were  observed sleeping at least once during six drives with a conditionally automated driving (CAD) system. Sleep is an illicit  driver behaviour in CAD because drivers must be available to take over vehicle control at system boundaries. However,  sleep was not only observed in the driving simulator environment, but nearly half of the participants indicated that they  intend to sleep during CAD in real life. CAD usage, gaze behaviour, subjective evaluation of CAD, trust and mental  model of CAD were compared for participants who indicated they intended to sleep in CAD and participants who  indicated no intention to sleep. The majority of participants understood that sleep is an illicit driver behaviour in CAD.  Participants with the intention to sleep used the simulated CAD more and they reported higher comfort levels during  CAD usage and perceived takeover situations as safer. Semi-structures interviews after the last drive indicated that  drivers would sleep during CAD once they had some experience with the system. The results suggest that drivers, after gaining experience with CAD, become complacent and sleep during CAD even though they know that it can potentially  lead to dangerous situations. Sleep during CAD is both automation misuse and automation abuse. Driver monitoring  systems for CAD must detect and prevent sleep in drivers.