Systems, devices, and methods for detecting ambient light with wearable heads-up displays are described. An ambient light sensor can be positioned close to a user's eye area on a user-side of a wearable heads-up display. By positioning the ambient light sensor on a user-side of the wearable heads up display, the ambient light sensor can be in a position and/or orientation which receives ambient light in a similar manner a user's eye, and thus ambient light detection can be more accurate. Brightness of display light output by the wearable heads-up display can be adjusted in response to the detected brightness of ambient light.