I know someone who bought earphones that shine light in you ears. According to what he was told, there are neurons that sense light and then make you feel wide awake when activated, which seemed like snake oil to me. Apparently the pineal gland may be able to sense light and it does secrete melatonin - a sleep regulating hormone. I'm still sceptical though as its stuck in the middle of your brain. Would shining lights in your ears be able to have any effect on how awake you feel?
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asked Jan 17, 2012 at 16:25
2 Answers
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There is no known mechanism for light detection through the ears in humans, as far as I know. It is certainly true that the pineal gland is part of the system that regulates the circadian rhythm (briefly, the daily sleep-wake cycle). However, while the pineal gland in birds and other non-mammalian vertebrates is directly sensitive to light, the mammalian pineal gland is not (see, for review, Doyle and Menaker, 2007 and Csernus, 2006).
In all animals, the circadian rhythm is regulated by a photoperiod cue and therefore requires light detection. In mammals, the light sensors are found exclusively in the retina, the sensory portion of the eye. There are two classes of light detecting cells in the retina. First, rod and cone photoreceptors mediate vision in the usual sense of the word. These cells contain proteins called opsins that absorb photons of light and thereby excite the photoreceptors that contain them, informing the brain that light was detected.
A second class of photosenstive cells in the retina are called intrinsically photosensitive retinal ganglion cells (ipRGCs) (see Do and Yau, 2010 for review). These cells mediate "non-image-forming" vision and are an important part of the circadian rhythm pathway. They also contain an opsin called melanopsin which is a photosensitive pigment. This is not to be confused with melatonin, which is the sleep hormone released by the pineal gland. The ipRGCs in the retina send the photoperiod cue to a brain area called the suprachiasmatic nucleus (SCN). The SCN then signals to the pineal gland.
If we are generous and assume that these light-emitting headphones are the result of misunderstandings, we can guess that the confusion arises from (1) the fact that some animals have a directly photosensitive pineal gland, but not mammals and (2) that the pineal gland secretes melatonin but not the photosensitive pigment melanopsin.
Update: From a bit of research, it turns out that the company selling the headphones is not "confused" as I politely offered. I don't think this site is the appropriate forum to refute their research or claims. Suffice to say that the retina is the only part of the human brain shown to be photosensitive.
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I believe there are light sensors (TRPV3) in the skin for infrared light (heat), that convey that information back to the brain from the skin. This is kind of light detection, but it is not direct detection like the rhodopsins in the eye.
By the way, without passing information onto neurons, cells probably have a lot of sensors they may use to respond to their local environment. This recent article talks about how olfactory receptors can be found in lung and gut cells. so its quite possible that the conventional light detecting genes (rhodopsins) would be found in skin cells, but they may not convey information to neurons.
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$\begingroup$ Can you provide a reference for a case in which a TRP channel, commonly thought of as a temperature detector, is plausibly used as a light sensor? $\endgroup$– yamadCommented Jan 18, 2012 at 0:14
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$\begingroup$ There has been one small study that seems to show that human skin is photosensitive. $\endgroup$ Commented Jan 20, 2012 at 13:35
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2$\begingroup$ The study I think you are referring to appeared to find that shining light on the back of the knee could entrain a human's circadian clock (Campbell and Murphy, 1998). That study has been refuted by a later study (Wright and Czeisler, 2002, see comment by Baringa, 2002). Wright and Czeisler show no response to light behind the knees. Current consensus is that mammalian photosensitivity resides only in the eyes. $\endgroup$– yamadCommented Jan 21, 2012 at 2:13
"extraretinal photoreception in mammals". $\endgroup$