The action spectrum for light induced melatonin suppression is known since 2001. The receptors being responsible for suppression of melatonin caused by the pineal gland were found in human eye. By using the melatonin suppression it is possible to describe and to influence one part of the circadian processes. Based on the experimental findings of BRAINARD and THAPAN it is possible to define the so called circadian action function c(lambda). With the help of this function GALL built up a circadian metric. This metric will be one topic of that report.