How Anti-sleep Alarms Work

Sensing Driver Sleepiness

An anti-sleep alarm would have already awakened this tired driver.
An anti-sleep alarm would have already awakened this tired driver.
Diana Starobinskaya/

Let's start with the simplest anti-sleep alarm system, the over-the-ear gadget. This lightweight plastic device has an arm that slips over one ear, like some telephone earpieces or portable headphones do. Once it's on, a sensor inside the case measures the angle from a perpendicular perspective. If the driver is looking straight ahead -- as he or she should -- the alarm measures the angle at zero degrees.

If you've ever fallen asleep on a subway train or in your high school algebra class, you know that your head tends to fall forward as you doze off. You also know that you may stay asleep for a few seconds or a couple of minutes before your head jerks upright and you're awake again. It's annoying on the bus commute home; it's downright dangerous when driving a car. The anti-sleep alarm looks for any indication that the driver's head is tipping forward: When the earpiece senses that the angle has increased from zero to, say, 15 or 30 degrees, it sounds an alarm. Most manufacturers stress that the sound is loud and irritating enough to wake the driver, but not so loud or sudden that he wakes up with a start and yanks on the wheel or steps harder on the gas.

You can easily adjust the angle that triggers the alarm. If you know you can sleep with your head almost upright, you can set your earpiece to a smaller angle. If the alarm goes off every time you bop your head along with the tunes on your radio, then you should set your triggering angle to 30 degrees or more.

While these alarms work to help keep drivers awake, some high-end manufacturers are adding sleep sensors to their cars right at the factory. A few notable systems:

  • Mercedes-Benz Attention Assist uses the car's engine control unit to monitor changes in steering and other driving habits and alerts the driver accordingly.
  • Lexus placed a camera in the dashboard that tracks the driver's face, rather than the vehicle's behavior, and alerts the driver if his or her movements seem to indicate sleep.
  • Volvo's Driver Alert Control is a lane-departure system that monitors and corrects the vehicle's position on the road, then alerts the driver if it detects any drifting between lanes.
  • Saab uses two cameras in the cockpit to monitor the driver's eye movement and alerts the driver with a text message in the dash, followed by a stern audio message if he or she still seems sleepy.

In-car systems can be expensive, especially those that involve in-dash cameras to monitor drivers instead of sensors that are already in place. Read on to find out who can benefit from an anti-sleep alarm, no matter what the cost.