The invention relates to a method and an illumination device arranged for reducing sleep inertia and/or for controlling alertness of a human being. The illumination device is arranged for reducing sleep inertia and/or controlling alertness by light radiation. The illumination device comprises one or more light sources for generating a first illumination output and a second illumination output. The first illumination output comprises light with a dominant wavelength in the range of 590-770 nm. The second illumination output comprises light with a dominant wavelength in the range of 400-560 nm. Thus, colored light is used. Alternatively, the first illumination output comprises white light with a color temperature below 4100 K, i.e. warm white light. The second color temperature then comprises white light with a color temperature above 4900 K. The illumination device comprises a controller configured for controlling the one or more light sources to expose said eye of said human being to the first illumination output during a first time interval and to the second illumination output during a second time interval. The second time interval terminates at a later point in time than the first time interval. The time intervals are selected such that at least one of the first and second time interval is less than 60 seconds.
