Light attenuation by the human eyelid.

Although it has been reported that light treatment during sleep can modify biological rhythms, the amount of light that is transmitted through the human eyelid has not been established. We evaluated eyelid transmission with a visual threshold response. Estimated light transmission through the eyelids was 0.3% for blue, 0.3% for green, and 5.6% for red light. The eyelid was an effective attenuator and acted as a red-pass filter. Illumination intensity and color balance after passing through the eyelid should be considered in evaluating the effects of light treatments during sleep.