Bright light entering our eyes increases our alertness and decreases sleepiness. This is the case during the day and during the night. In the morning, when the eyes are still adapted to darkness, relatively moderate light intensities (such as the ones of the Wake-up Light) can be sufficient to activate the body and reduce sleepiness. During the day one needs to seek much brighter light to increase alertness, such as the light provided by the sun or Philips light therapy devices.
In the case of the Wake-up Light, the gradual increase of light intensity before awakening gently prepares the body for waking up and has the additional effect of reducing sleepiness.