Does the colour of light affect the internal clock in humans?

Ambient light does not only allow us to see, it also influences our sleep-wake rhythm. A study in mice in 2019 suggested that yellowish light has a stronger influence on the internal clock than bluish light. In humans, the main effect of light on the internal clock and sleep is probably mediated via the light-sensitive ganglion cells. To understand the effects of the different light stimuli on humans, in the sleep laboratory the researchers determined whether the internal clock of the participants had changed depending on the colour of the light. Additionally, they assessed how long it took the volunteers to fall asleep and how deep their sleep was at the beginning of the night. They found no evidence that the variation of light colour along a blue-yellow dimension plays a relevant role in the human internal clock or sleep. The study found that the human circadian clock is relatively insensitive to shifts in light colour towards warmer colour temperatures at constant melanopic illumination.