Your eyes are very much involved in your sleep patterns – way beyond the physical involvement of opening and closing your eyelids. Rods are cells in our eyes basically detecting light. Our brain and our body respond to the light detected by our eyes. When we are active and when we sleep is our so-called circadian rhythm and this rhythm is influenced by the light/dark cycles of external lights from the sun and the artificial lights we use indoors. Our eyes’ response to the light can be observed during the dilation and constriction of our pupils.
The light to brain function is not performed by rods and cones alone. Researchers have identified a small subset of retinal ganglion cells (RGCs) which are intrinsically photosensitive, thus are called ipRGCs and are responsible for maintaining our body clocks (circadian rhythms) as well as the pupil constriction and dilation. These cells produce a protein called melanopsin that allows them to sense light on their own and send information about light intensity to the brain.
Yet, the ipRGCs are rather slow compared to the light-detecting function of rods and cones. They capture very little light but, once captured, the light is very effective in producing a signal to the brain. The researchers found that the density of melanopsin in ipRGC membranes is almost 10,000-fold lower than that of rod and cone pigments. But once the threshold is reached, the signal is sent to the brain efficiently enough. So how can this rather slow signalling mechanism be useful? In fact, the slowness of the ipRGCs is what makes them useful.
Study author Dr. King-Wai Yau at Johns Hopkins University School of Medicine explains:
“In terms of controlling the pupils and the body clock, it makes sense to have a sensor that responds slowly and only to large light changes. You wouldn’t want your body to think every cloud passing through the sky is nightfall.”
Those of you who have had experienced jetlag would know that the body takes time to recover as the ipRGCs in your eyes adapt to a new light and dark pattern.
This discovery of ipRGCs and how they work can help in unravelling some of the mechanisms not only of jetlag but also other sleeping disorders such as insomnia, and light-triggered neurological disorders such as seasonal affective disorder (winter blues).