Light, especially the blue-light from electronic devices, disrupts sleep. Anyone who's experienced jetlag understands that human beings have an internal circadian clock that determines everything from energy and hunger to glucose tolerance and muscle building. Even without light or darkness, our body knows when it should be awake and when it should be asleep.
The eyes are the master of this clock because of their ability to sense light. When there are low levels of light, special nerve cells communicate to our brain that it's time to get ready for bed. The brain then releases chemicals to help us wind down and the pineal gland releases a hormone called melatonin. Melatonin is the yin to the yang pumped out by the adrenal glands.
Since light determines whether or not we get the message to secrete melatonin, it's easy to see how staring at an electronic screen, or simply being in a brightly-lit house in the evening, can be problematic. Your brain knows it's time for bed, but it's still waiting for darkness so it can cue the pineal gland to secrete melatonin and start down-regulating stress.
Blue light, from electronics in particular, disrupts melatonin production, which means we'll continue to suffer from poor sleep and a disrupted circadian cycle if smart phones, TVs, tablets, and computers continue to be our main source of entertainment in the evening. Unfortunately, white light from a light bulb or overhead fluorescent appears to be nearly as problematic as blue light as research reveals that short durations (1-2 hours) cause near-daytime melatonin levels after minimal exposure (500-1000lux).
Some people even use one capsule of Z-12™ (instead of 2-3, the normal dosage before bed) for daily stress reduction.
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