A wearable head-mounted device for delivering photobiomodulation (PBM) therapy using ambient light is disclosed. The device includes a headwear structure and at least one optical element configured to receive incident ambient light, convert at least a portion of the ambient light into one or more wavelengths within a photobiomodulation range, and re-emit the converted light toward a user. The optical element may operate passively without requiring electrical power and may include quantum dots, phosphors, or other wavelength-converting materials. The re-emitted light may include wavelengths between approximately 600 nm and 1000 nm and may be directed toward the a scalp, a face, a neck, or eyes of a user. The device may be implemented in headwear including hats, caps, or visors to provide therapeutic light delivery.