Our biology is set up to work in partnership with the sun. From our sleep cycles to our immune systems and our mental health, access to sunlight is crucial for living a happy and fulfilling life. New research suggests that our sun exposure over a lifetime – even before we were born – may shape our risk of developing a range of different illnesses, from depression to diabetes. Geddes explores the extraordinary significance of sunlight, from ancient solstice celebrations to modern sleep labs, and from the unexpected health benefits of sun exposure to what the Amish know about sleep that the rest of us don’t.