Researchers from the University of Oxford find that the specific wavelength of light hitting neurons at the back of the eye causes mice to fall asleep faster, slower or for longer. They also identify the pigment responsible: Melanopsin.
In a society where both work demands and social activities chronically impact the quality of our sleep, understanding the implications of sleep deprivation is of great value. Basic research into the nature of sleep in mammals will allow us to better understand sleep physiology and what we can do to ensure we get enough quality sleep. In this vein, Pilorz et al., have found that mice fall asleep more quickly or slowly, and for different lengths of time, depending on the colour of the light hitting special cells at the back of the eye.
Photosensitive retinal ganglion cells (pRGCs) are neurons that are involved in non-image forming photoreception, and are known to express the photopigment melanopsin. Pilorz and colleagues set out to elucidate the role of melanopsin regulation of sleep-inducing and arousal-promoting effects of lights of different wavelengths in mice. This study elegantly examined the role of melanopsin under the influence of light on sleep duration and induction, on both the physiological and molecular level. The first step was to look at how different wavelengths of light can produce differential effects on the behavioural output in mice.
The authors found that green light, at a wavelength of 530 nm, produced a very rapid sleep onset compared to blue light (470 nm) in which onset was delayed. Exposure to blue light also resulted in a reduction of total sleep duration.
When the same experiments were performed in a melanopsin knockout background: where melanopsin has been genetically removed from the animal, the phenotype was reversed. Blue light caused rapid loss of consciousness, and green light extended the duration of sleep. Further, when wild-type (melanopsin proficient) mice were placed in a chamber split into a dark zone and an illuminated region, the duration spent in the dark zone was significantly higher in the presence of blue light as opposed to green. This is indicative of anxiety and could be related to the delayed sleep onset in the presence of blue light.
Once again, under knockout conditions of melanopsin, no difference in duration was observed under the exposure of either blue or green light.
These results highlight the intrinsic ability for mammals to respond to atmospheric light, in particular through the photopigment melanopsin. In humans, the effects of sleep deprivation can be obvious to the individual on a daily basis, through fatigue and lack of cognitive abilities. A lack of sleep has also been directly linked with some of the most common leading causes of death, including heart disease and diabetes. Knowing that simple lighting conditions have such a profound influence on sleep via a distinct retinal receptor broadens our understanding: not only is the presence or absence of light important, but so is the wavelength. And now we’re starting to understand why.
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Written by Fuad Mosis