A system configured to monitor ambient illumination experienced by a subject. In one embodiment, the system comprises an illumination sensor, a timer, and a storage module. The illumination sensor is configured to monitor an intensity of ambient illumination within two or more wavelength ranges by generating one or more output signals that convey information related to the intensity of ambient illumination within the two or more wavelength ranges. The timer is configured to indicate the passage of periods of time. The storage module is configured to store information related to the intensity of ambient illumination within the two or more wavelength ranges, as conveyed by the one or more output signals, for individual periods of time. The system is portable to be carried by the subject.