53. Light therapy: white, blue or maybe green?

30 May 2010 at 14:00 | Posted in Circadian rhythm | 3 Comments
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It has long been known that light and dark affect the daily as well as seasonal rhythms of living things.  Early in the 1900s it was assumed that humans’ daily rhythms were less affected than those of “lower” beings, but that attitude was proved wrong.

In the 1980s it was noted that some totally blind people entrained perfectly to the 24-hour cycle while many did not.  Until some time in the 1990s, it was not known if entrainment occurred by light to the eyes or to the skin.  One research team claimed to have proven that light to the backs of the knees effected entrainment, but neither they nor other researchers could duplicate those results, which later were withdrawn.

It is now clear that we and other animals entrain primarily by light to the eyes, though secondary cues such as social activity, feeding times etc. also play a role.  It is also clear that our eyes contain not only rods and cones for vision but also the recently discovered light sensitive ganglion cells for the entrainment of circadian rhythms.  The light sensitive pigment in these cells is melanopsin.

There it stands, but there are always new questions to be answered.  One is: 

Do the light sensitive ganglion cells contribute to vision,

and do the rods and/or cones contribute to

the non-visual effects of light?

If the ganglion cells’ photosensitivity contributes to vision at all, it is very minimally.  But a study* published this month by well-known researchers suggests that the cones do affect entrainment, with variations dependent on the timing and intensity of the light.  The practical implications include the question of whether the use of blue-blocking goggles in the evening, so-called “dark therapy”, really does allow normal flow of melatonin as intended.  [An aside: mightn’t there be a more direct way to find this out?]   In addition, the jury is still out on what color light – white, blue or perhaps green – is best to use in light therapy.

In this study in Boston, more than 50 human subjects each spent 9 days in an laboratory environment free of time cues.  Semi-recumbent, totally confused about the time of day, and with their melatonin rhythms individually determined, half of them were exposed to blue (460nm) light and half to green (555nm) for 6.5 hours starting shortly after their own melatonin secretion started. 

It is well-known that blue light (including about 460 to 482nm) excites melanopsin leading to the suppression of  melatonin.  The question here is whether the green light may have similar or equal effects. Green was chosen because the human visual system is most excitable at green (555nm).

In the figure, A and B are two subjects.  Each person’s normal pattern of melatonin secretion is repeated on the left and on the right in black.  On the left, blue light acutely suppresses A’s melatonin, while green light only delays B’s for a good hour.  The next night, as we see on the right, the pattern of melatonin secretion has been phase-shifted by the previous night’s light exposure about equally for the two subjects (horizontal red line).  From previous knowledge, one would have expected an appreciably greater delay after the blue light than the green.

The authors write: 

“Our data … raise the possibility that activation of cone photoreceptors in the late evening by relatively low-illuminance light sources, such as liquid crystal display monitors, table lamps, and dimmable lamps, may delay the circadian clock and therefore contribute to the high prevalence of delayed sleep phase disorder.” [My emphasis.]


“[B]locking short-wavelength light with blue-blocking goggles may not always be effective in preventing undesired circadian responses based on our finding that longer-wavelength light is able to induce robust phase-shift responses.”


“Our findings have implications for the development and optimization of light therapies for a number of disorders, including circadian rhythm sleep disorders.…”

* Spectral Responses of the Human Circadian System

Depend on the Irradiance and Duration of Exposure to Light.

Joshua J. Gooley, Shantha M. W. Rajaratnam, George C. Brainard,

Richard E. Kronauer, Charles A. Czeisler and Steven W. Lockley

Science Translational Medicine, 12 May 2010 

See also these older posts: 

xliii. Blindfolding the blind   

xliv. Rods and cones and the “new” ipRGC 

 (posted by D )


Next post:  #54. Take a Nap!




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