Wear behind the right ear it works on the principal of electronic balance. As the drivers head nods below a preset angle, it emits aloud alarm signal alerting the driver to take corrective action, take a break, or even take a nap. The nap zapper is not only designed for drivers, it is perfect for people who need to be on full alert while on duty, such as security guards, machine operators and students who need to work in their posture.