The present invention relates to a portable illuminating device (110) comprising at least one illuminating light source (112) emitting light of a wavelength comprised in the range of 450 to 495 nm, preferably, comprising at least one further illuminating light source (112) emitting light of a wavelength comprised in the range of 620 nm to 750 nm； a diffuser (113) scattering the light of said illuminating light source (112) at least in part in the direction of at least one eye, preferably at least one cornea and/or retina, of said subject； and a mounting mechanism (114) for fixing the portable illuminating device (110) to the body of a subject. Preferably, said diffuser (113) scatters the light of said illuminating light source (112) at least in part in the direction of at least one eye and in the direction of at least a part of the facial skin of a subject. Preferably, the portable illuminating device (110) further comprises a sensor for determining if illumination is advisable, preferably a melatonin sensor, and/or comprises an oxygen and/or pulse sensor. The present invention further relates to the portable illuminating device (110) for use in preventing and/or treating a disease or disorder caused by or associated with a distorted circadian rhythm； and to a method of improving physical and/or mental performance of a healthy subject, comprising the step of illuminating at least one cornea of said subject with light comprising light of a wavelength of 450 to 495 nm for at least 15 min, thereby improving physical and/or mental performance of a healthy subject. Preferably, the invention relates to a method of improving physical and/or mental performance of a healthy subject, comprising illuminating at least one eye and/or at least a part of the facial skin of said subject with light comprising light of a wavelength of 450 nm to 495 nm and/or illuminating said subject with light comprising light of a wavelength of 620 nm to 750 nm, thereby improving physical and/or me
