In one interesting study, 12 adults were randomly assigned reading from a light-emitting eBook for 4 hours before bed or reading from a print book for 4 hours before bed for 5 consecutive evenings. They found that those reading an LE-ebook displayed decreased subjective sleepiness, decreased EEG delta/theta activity, suppressed melatonin secretion, lengthened sleep latency (increased time to fall asleep), delayed and reduced REM sleep, and impaired morning alertness.3
Why does blue light have this effect? Human eyes have rods, cones, and intrinsically photosensitive retinal ganglion cells. Rods and cones are mostly responsible for image-forming vision. Intrinsically photosensitive retinal ganglion cells are responsible for regulating circadian rhythm and other biologic functions. Retinal ganglion cells form the beginning of the retinohypothalamic tract which transmits light data from the retina to the hypothalamus to regulate circadian rhythms throughout the body. Retinal ganglion cells respond powerfully short-wavelength light like the blue-colored light emitted from most devices and LED televisions. This blue light will cause a high rate of fire from the eyes to the hypothalamus, which will change hormonal and circadian rhythms.4 Blue light also has the ability to stimulate other areas of the brain which are responsible for producing norepinephrine, a neurotransmitter that has potent arousal properties.
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