Russell Foster talks about sunlight and health

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.