Color change of the plaice and frog, and development of the testes in duck are induced by light stimuli via the eye. Eliminating experiences on the frog (Hollwich) and on the duck (Benoit) showed that these stimuli – beside vision – use a separate way, the so-called ‘energetic portion’ of the visual pathway (Hollwich). In compared studies on blind and on temporarily blind patients involved by cataract in both eyes, we found significant low levels of ACTH and cortisol. The levels normalized after cataract extraction. – Furthermore, we studied the influence of strong artificial illumination (3,500 and 3,200 lx) on healthy persons with considerable and, on the other hand, with minimal difference of their spectral composition to daylight. We found stress-like levels of ACTH and cortisol in the first group. These findings explain the agitated mental and physical behavior of children staying the whole day in school under artificial illumination with strong spectral deviation from daylight. Therefore, to avoid mental and physical alterations, the spectrum of artificial illumination should be largely similar to that of natural light.