For these mice, the colours had opposite effects - blue caused rapid sleep onset, while green and violet significantly delayed sleep, showing that melanopsin is necessary for the substantial wavelength-dependent effects of light on sleep. To investigate the role of melanopsin, the team performed the same test on mice lacking the pigment. We confirmed the effect by testing mice using green and blue light at a time when they would usually be less active.' Blue and violet light delayed sleep - the onset of sleep taking between 16 and 19 minutes for blue and between 5 and 10 minutes for violet.ĭr Peirson said: 'The results meant that mice exposed to blue light had less sleep than those exposed to violet and green light. However, it was green light that produced rapid sleep onset - between 1 and 3 minutes. Based on the existing data about the role of melanopsin in sleep, they expected that the blue light would induce sleep fastest as the wavelength of the blue light (470 nanometres - nm) was closest to the peak sensitivity of the pigment (around 480nm). The team exposed mice to three different colours of light - violet, blue and green. We wanted to understand how these two effects were related and how they were linked to a blue light-sensitive pigment called melanopsin, known to play a key role in setting our body clock.' Yet, at the same time, it also increases levels of corticosterone, a stress hormone produced by the adrenal gland that causes arousal - wakefulness. The researchers, led by Dr Stuart Peirson from Oxford's Sleep and Circadian Neuroscience Institute were aiming to understand why exposing mice to bright light caused two - physically incompatible - responses.ĭr Peirson explained: 'When we expose mice to light during the night, it causes them to fall asleep.

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