Melatonin production starts ramping up about two hours before your body’s natural sleep cycle would start—often described in research as about 10 p.m. in local time. And it’s produced in the largest quantities in the wee hours, wherever in the world you are, right in the middle of what your body perceives as the darkest time of day.
If you want to sleep better, the almost universal suggestion from both sleep and lighting researchers is to turn off any screen two hours before your planned bedtime.
Since this light receptor type was discovered, scientists have connected in many, many studies not just light and melatonin suppression, but specifically light that’s heavy on blue frequencies. Blue light can ostensibly offset the cycle of hormone production by a couple of hours or more. This has led to speculation that staring at television sets, monitors, and mobile displays disrupts or delays sleep. If you have to get up or are woken up at a fixed time, as for most people, this both reduces sleep and throws off the body’s endocrine and other systems.
Daylight has a large proportion of short-wavelength light at the blue end of the spectrum (around 460 nanometers). Indoor lightning has been traditionally “warmer,” or towards the yellow, longer-wavelength end (about 555 nm) or red at the far end (650 nm). That’s true of fire and most incandescent lighting.
But a shift in lighting over decades has shifted towards cool, “white,” or “daylight” illumination, whether incandescent, fluorescent, or LED. While thought of as whiter, they actually produce bluer light, resembling more closely our perception of a sunlit day.
This description of color gets labeled color temperature, and is measured in kelvins (K). On one end of the spectrum, you have red/yellow candlelight at 1,000K, considered very warm; at the other end, pure blue sky is 10,000K, considered very cool.
Most LCD monitors and mobile displays can calibrate against a standard called D65, which centers at 6,500K, fairly blue and fairly cool—it’s described as outdoor daylight at noon. Many displays are tuned or default to a higher temperature, though, and are much bluer.
Specific research and reasonable speculation centers around how predominantly blue light from television sets, computer monitors, laptop displays, and mobile screens might be connected with the increase of a host of ailments in nations in which a large percentage of residents use those technologies before and at bedtime.
Of special interest is the simple lack of sleep. The CDC estimates 50 to 70 million U.S. adults have disorders that prevent them sleeping sufficient to be alert, productive, and rested on an average day.
All the discussion of blue light has led to programs and extensions for many computer platforms that attempt to reduce the production of blue light in order to avert circadian rhythm disruption. The f.lux software is a well-known example, available for OS X, Windows, Linux, and rooted Android phones. (It could be installed through a work-around in iOS, until Apple asked f.lux to stop distributing it.)
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