Brother sun and sister moon

At the very back of the eye is a cluster of cells called ipRGCs. They are cells that don’t depend on vision to sense light, and that keep the circadian rhythms of both sighted and non-sighted people in sync with the sun. Without them, we would not feel the pull of sleep at night; we might fall asleep in the middle of the day and we would feel perpetually jet-lagged. It is the continuous effect of the strong, high-lux light of the sun and dim, low-lux light of the moon on these cells that keeps us, essentially, in sync with time and with society.

This is too true for Linda Geddes’s interviewees in Chasing the Sun: The New Science of Sunlight and How it Shapes our Bodies and Minds (Wellcome, £14.99). Mark has a sleep disorder that means he sleeps incrementally later every night, putting him gradually more and more (and eventually less and less) out of sync with the sun.