We have light receptors in rods and cones of our eyes – they play a role in our vision. However, there is another light receptor called melanopsin, and it doesn’t help us see better. It just helps our brain understand whether it is sleep time or wake time. You may have not heard about melanopsin before – and this might be because it was first discovered less than 20 years ago!
People who are blind still react to light. For example, when exposed to darkness, the level of ‘sleep hormone’ melatonin increases in their bodies, and vice versa. They, however, might not be consciously aware of their light/dark surroundings.
Melanopsin regulates our circadian rhythm – that is, when we wake up, are fully alert, hungry, and sleepy.
Light receptors are proteins which absorb light. They are in the eyes and are the first to receive light and send it out to other cells. Thanks to them, our minds can ‘solve the puzzle’ of brightness, shapes, and colors. Light receptors are also called photoreceptors.
In the retina of the human eye, there are three types of photoreceptors, called opsins – rhodopsin (in rod cells, ensuring night vision), photopsin (in cone cells, giving us color perception) and melanopsin (in neurons of the retina, taking part in circadian rhythm regulation and pupil response to light).
The cells in which melanopsin is found are called intrinsically photosensitive retinal ganglion cells (ipRGCs), or melanopsin-containing retinal ganglion cells (mRGCs).
Melanopsin is a photopigment found in cells of the mammalian eye, but it has no role in sight. It is involved in the circadian rhythm and the expansion and narrowing of the pupil (pupillary light reflex).
In 2002, a statement was published saying that numerous studies report the discovery of melanopsin and cells which detect light.
The human melanopsin gene is called OPN4. It is expressed in the previously mentioned group of the retinal receptors – ipRGCs. These cells make up about only 1-2% of retinal nerve cells.
Melanopsin is the most sensitive to the blue light spectrum. What ‘sensitive’ means is that this protein, when exposed to the blue light, takes a certain shape from which it can produce an electrical signal (action potential). Light from the orange-red spectrum is what brings it back to the blue-sensitive state.
Some studies suggest that melanopsin has a role in cognition because the blue light (received by melanopsin-containing cells) causes activity in the brain regions associated with memory.
There is still a lot left for scientists to discover – there has even been clinical research about how melanopsin could be used in treatments of different human problems, one of which is – sleep/wake cycle problems.
Melanopsin – functions and how it works
Cells that contain melanopsin adapt to light, but also to darkness. This is also seen in rod and cone cells. All photoreceptors react to the recent lighting background. If light is not present, they become adapted to dark and thus more sensitive to light and vice versa.
Melanopsin photoreceptors are not very sensitive to short wavelength light (close to the violet spectrum) and are the most sensitive to the blue light spectrum.
Neural pathways from melanopsin cells to the brain areas
Our eyes detect light regardless of whether they are open or closed. As soon as light hits the eye, melanopsin cells send out nerve impulses (electrical signals). They travel through nerve cells which lead to different target areas.
These signals, informing about light, arrive in the midbrain, which then reacts by resizing the pupil. They also send signals to the hypothalamus, which regulates circadian rhythm. However, before the information reaches the hypothalamus, the signals from all three opsins are gathered together.
Sleep-wake cycle and melanopsin
Here we’re going to see how exactly melanopsin influences our circadian rhythmicity and how it regulates our sleep.
Melanopsin and the circadian rhythm
Our circadian rhythm is dictated by the internal biological clock. For example, one person could always go to bed at, say, 11 pm and wake up at 7 am, be the most productive before noon and the least in the evening – all of these things are dictated by their circadian rhythm.
In our brain, there is an area called the hypothalamus, and in the hypothalamus, a small nerve cell population, the suprachiasmatic nucleus (SCN), is also referred to as ‘the master circadian oscillator’ – because of its important role in making the circadian rhythm.
These ‘master cells’ need input from the outside world – they need to know when to prepare the body for sleep and when to ensure the energy boost. Melanopsin comes in here. The melanopsin cells (ipRGCs) receive the light and fire the electrical signal which goes to SCN.
This allows our body to adjust its rhythm to that of the outside world, so our sleep needs could change with the season, and luckily, we are able to synchronize with a new time zone after traveling long distances. This ‘adjustment’ is called entrainment, or more specifically, photoentrainment.
Melanopsin also seems to communicate with dopamine, regulating retinal clock gene expression.
Melanopsin and sleep
We know that both daylight and artificial light have strong effects on our sleep – both positive and negative.
Plenty of sunlight is good for your circadian rhythm and makes it easier for the body to release chemicals that prevent arousal and make you fall asleep. On the other hand, spending time indoors sends a confusing message to SCN and artificial lights at night (especially LED lights) have proven over and over to keep us awake by delaying sleep onset.
Melanopsin cells tell the hypothalamus that we’re exposed to light. As we already know, they are sensitive to the blue light (found in the majority of screens and LED lightbulbs), and if these cells pick up light stimulus all evening, then the hypothalamus won’t order the production of ‘sleep hormone’ melatonin.
This is why, if you crave a good night’s rest, it’s best to shut off all electronics and dim the lights to allow your body to adjust to the fact it’s dark and time for bed.
People who have low melanopsin levels frequently suffer from sleep (and mood) disorders. Another recurring problem they may experience is the seasonal affective disorder (SAD) which has been described as sleep-mood regulating hormones go through imbalances as a consequence of insufficient daylight.
Now it is obvious that people with low levels of melanopsin are more likely to suffer from SAD simply because their cells register less light.
They have no sight problems, however. Their problems lie in the neural connections between the eyes and the hypothalamus, which regulates hormonal secretion – and hormonal imbalances are known to be in a tight relationship with sleep disorders.
Melanopsin and the color of the light
One recent study published in the PLOS journal stated that melanopsin plays a role in sleep and wakefulness, but the interesting bit is that the researchers used different colors of light to see the reaction in mice. What they found is that blue light was promoting wakefulness because it increased sleep-delaying stress hormones, whereas the green sped up the process of falling asleep.
These lights weren’t effective on mice without melanopsin which means melanopsin was the most important link in the process of light affecting alertness.
The clinical use of melatonin to help with sleep/wake patterns
Scientists have managed to use the blue light impulses to manipulate with the brain activity of genetically modified mice to wake them up from deep sleep (slow-wave sleep).
Scientists believe a similar kind of light exposure can be used to control the sleep-wake cycle in humans because blue light promotes wakefulness and suppresses sleepiness.
- Sengupta, A. Melanopsin: A Photopigment Regulating Circadian Photoentrainment May Lead to a Blue Light-Induced Treatment of Diabetes. http://photobiology.info/Sengupta.html Accessed February 18, 2019.
- Paul K.N, Saafir T. B, and Tosini G. The role of retinal photoreceptors in the regulation of circadian rhythms. Reviews in Endocrine and Metabolic Disorders. December 1, 2010. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2848671/ Accessed February 18, 2019.
- Altimus C. M, Güler A. G, et al. Rods-cones and melanopsin detect light and dark to modulate sleep independent of image formation. Proceedings of the National Academy of Sciences of the United States of America. December 5, 2008. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2596746/ Accessed February 18, 2019.
- Wong K. Y, Dunn F. A, Berson D, M. Photoreceptor Adaptation in Intrinsically Photosensitive Retinal Ganglion Cells. Neuron. December 21, 2005. https://www.cell.com/neuron/fulltext/S0896-6273(05)00964-5? Accessed February 18, 2019.
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