Part 9 of Joy and Tranquillity digital guide

Russell Foster talks about sunlight and health

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The environment exposes our bodies to an endless stream of sounds, smells, tastes, physical contact, heat and, of course, sunlight. We use the energy from sunlight to synthesise vitamin D in our skin. Vitamin D is really important, as it helps us build strong and healthy bones. But it’s also involved in how we respond to infection, the growth of new cells, muscle contraction and metabolism, and most recently, low levels of vitamin D in the blood have been linked to depression.

But sunlight provides us with so much more than just vitamin D. We sense light not through our skin, but by using our eyes, and a layer of cells in the eye called the retina. The rods and cones of the retina act as light detectors or photoreceptors, and the light reflected from objects in our environment allows us to build an image of our world – and we call this “vision”.

In addition to providing us with vitamin D and our sense of vision, sunlight also provides us with a sense of time. Each of us has an internal biological clock which we use to wake us up or send us to sleep, and to prepare our bodies generally for the different demands of activity during the day and sleep at night.

These 24-hour biological rhythms are called “circadian rhythms”, and they need to be adjusted each day to the daily cycle of sunrise and sunset.  And it’s the same for jet lag. When we fly across multiple time zones and suffer jet lag, we eventually recover from jet lag because sunlight at dawn and dusk resets our circadian rhythms to the new time zone. Sunrise and sunset is detected by the eye, but not by the visual cells – the rods and cones. There is another, recently discovered, light-sensing cell in the eye called a “photosensitive retinal ganglion cell”.

Without this daily resetting, our circadian rhythms would drift out of synchrony with the 24-hour day, and this mismatch leads to multiple health problems, including depression and, in some vulnerable individuals, severe mental illness.

About the speaker

Photograph of Russell Foster

Russell Foster

Russell Foster CBE, FRS is the Head of Oxford’s Nuffield Laboratory of Ophthalmology, the founding Director of the Sleep and Circadian Research Institute (SCNi) and is a Fellow of Brasenose College Oxford. His research addresses how circadian rhythms and sleep are generated and regulated and what happens when these systems fail because of societal pressures, ageing and disease. A key finding has been the discovery and characterisation of an unrecognised light-detecting system within the eye that regulates circadian rhythms and sleep. For his work, Russell was elected to the Fellowship of the Royal Society, the Academy of Medical Sciences, and was honoured by being appointed as a CBE for services to science. He has been a member of the Governing Council of the Royal Society and he established and led for six years the Royal Society Public Engagement Committee. He is also a Trustee of the Science Museum Group. Russell has published over 270 scientific papers and has received multiple national and international awards. He has also written four popular science books and will publish his fifth in March 2022.