Research by scientists at The University of Manchester has revealed that the colour of light has a major impact on how the brain clock measures time of day and on how the animals' physiology and behavior adjust accordingly. The study, for the first time, provides a neuronal mechanism for how our internal clock can measure changes in light colour that accompany dawn and dusk. In research publishing on April 17th in the Open Access journal PLOS Biology, the researchers looked at the change in light around dawn and dusk to analyze whether colour could be used to determine time of day. Besides the well-known changes in light intensity that occur as the sun rises and sets, the scientists found that during twilight light is reliably bluer than during the day.