OS X Mavericks Uses Mac’s Light Sensors To Prevent System Sleep


A coder has found out that OS X Mavericks uses a Mac computer’s ambient light sensors to detect motion, subsequently preventing the system’s Energy Saver sleep functions from activating, The Verge reports. It was initially thought the feature was using the iSight camera to monitor movements but was later discovered to actually be using the Mac’s ambient light sensors.

Energy saver

In routine, the light sensors are used to adjust screen brightness to ambient light, but it appears that OS X Mavericks puts the same sensor to a different purpose i.e tracking changes in the light as “movement”.

“A coder turned up a surprising feature in OS Mavericks, a chunk of code that seemed to reset idle time whenever the computer sensed movement. Immediately, the concern spread to Twitter. Did that mean the iSight camera was always on, opening up the same privacy concerns that have dogged the next-generation Kinect? Tests seemed to confirm the suspicion, when moving around in front of the computer was shown to stave off sleep mode. As long as the computer sensed movement in front of the screen, it wouldn’t go idle.”

Our own tests have also confirmed that covering the camera but not the light sensor, sleep mode can be delayed by changing the ambient lighting conditions.

What do you guys think of this neat trick in Mavericks?


  • Sounds like a neat idea, but I don’t like that it would be doing this without my knowledge. If I set my computer to go to sleep after 5 minutes, I want to know that it’s going to sleep after 5 minutes, especially if I’m counting on a password protecting it from waking. If there’s something that might change that behaviour, I want to know about it. I think it’s an interesting feature but it should be a checkbox that you select, not automatic.

    There are times when I’m away from my computer, but it’s sitting beside a window that faces a busy street. Sunlight can reflect off the passing cars into my window and I wouldn’t want my laptop to avoid going to sleep because of those constant light changes.

    Something that would be more useful, perhaps would be if it could sense when I’m touching the laptop. There are many times when I read or watch something without moving much, but my hand is usually on the corner of the laptop or on the mouse. It would be nice if it could detect that and keep the laptop from sleeping while it senses my presence, although that would require extra sensors and use a bit more battery life, so I’m not sure that would be worth it.

  • david murray

    How about if the machine made an audible ‘yawn’ that you could respond to with something like, “Stay awake Mac” 🙂

    Seriously, my Mac kept falling asleep today while I had it displaying a project plan on one wall and I was white-boarding on another wall. Although I suppose a ‘yawning’ sound during my presentation might not be so good either …