None the less, the wavelength of the light around us has affected us since the dawn of life, and it still does. Foster, who researches the effect of light on sleep, tells me that our circadian rhythms – our “biological clock” – are profoundly affected by not just the brightness of the light we’re exposed to, but the colour of it. He was behind the discovery of a previously unknown cell in the optic nerve which acts as a sort of photon-counter, keeping track of how much light has hit it in the last few minutes. It is especially sensitive to blue light – specifically, the blue of a blue sky. If you’re exposed to light of this colour, it will make you more alert. “Blue light keeps us awake far more effectively than red light,” he says; “there are apps, now, that change your lighting before you go to bed, to get you ready for sleep.”