The invention relates to a method for controlling the alertness of a human subject and a light source for use in this method and use of a light source for this method. The method comprises exposure of a human subject during an exposure period to suitable light radiation for suppression of a melatonin built-up or stimulation of a melatonin degradation. Melatonin is a sleep-hormone that can be used to control the alertness of a human subject. The suitable light radiation being specified by an output fraction of melatonin suppressive radiation (Melatonin Watt/Watt) and light output (lumen/Watt), the output fraction and light output being adjusted to obtain the desired effect on the phase of said cycle.
