If you are changing your outdoor light bulbs, it's no longer a matter of swapping out the old incandescent bulb with a new one. Take a moment to select the best available option instead of simply what a store puts on prominent display. For several important reasons, a Nobel laureate would want a soft white LED light rated 3000K or less. The other option--the garish blue-rich lights similar to LED car headlights--may be inflicting harm on you, your neighbors, and the natural environment.
The Nobel Angle
Question: How many Nobel Prize laureates does it take to screw in a light bulb?
Answer: None, but if you're smart you'll consider the work of two recent prize winners.
The 2014 Nobel Prize in Physics was awarded to a team "for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources".
"That's great," you may say, "LED lights win the prize for being so energy efficient. That's the bulb I want." Yes, but not any LED, and especially not the blue-intensive LED's that are sometimes dubbed Daylight or Cool by the manufacturers.
Which brings us to the 2017 Nobel Prize in Medicine, awarded to a team "for their discoveries of molecular mechanisms controlling the circadian rhythm." The circadian system regulates body functions based on time, and the winning discoveries give us a tool for addressing issues related to day and night. Living things respond genetically to signals they receive that pertain to the clock, such as that light you are projecting outward at night. We are biologically honed to respond to that blue-intensive light.
A Simplified Primer on Circadian Evolution
The body performs certain functions at certain times of the day. For example, we are most alert in the morning, but take a dip in the early afternoon. Siestas are genetically recommended. At night, body temperature drops, and only in the absence of light does the body produce melatonin.
To set its clock, the body has to determine the basic facts--when is it night and when is it day? Inside your retina, in addition to rods and cones, are light-sensitive cells that detect incoming light and send signals to the suprachiasmatic nucleus in the brain, simplified as the SCN. The SCN is like the orchestra conductor of all things related to time. When the SCN detects blue light, its evolutionary response is to say, "Blue equals daytime sky." Whether from a blue sky or simulated blue light, the SCN triggers daytime functions. In the absence of light the SCN responds with "Dark equals nighttime," and puts your bodily functions in nighttime mode.
When the blue light goes away, the body thinks the sun has set. The SCN responds: Give the body some twilight time, then go to sleep. At least, that's how we evolved over millions of years. Suddenly, in the modern era, we send blue-light signals to our brain suggesting it's still or just recently daytime. Think of TV screens and computer monitors at night. Switch them off late and you're laying awake at night while your body says, "The sun just went down. Go to sleep in another hour." It's no wonder such misalignment of the circadian system with the natural order produces sleep phase disorders. [Note: you can install an app on your computer to take out the worst of the blue light emitted by your monitor at night. See https://www.nightwise.org/single-post/2015/08/19/Lessening-the-Blues.]
Sustained exposure to bright light will shut off the production of the hormone melatonin, which suppresses some cancers. The body makes melatonin only when the SCN thinks it is nighttime. So if your nighttime mode never kicks into gear because of artificial lights, and you make no melatonin as a result, you are susceptible to the woes that melatonin normally combats.
The circadian system is most sensitive to what the SCN deems to be daytime--light with a wavelength around 450 nanometers, which is in the blue part of the spectrum. Unfortunately, those blue-rich LED lights happen to put out light exactly in that same critical part of the spectrum--around 450 nanometers. That is, the bluish lights hit us right where our circadian system is most sensitive.
Check out the excellent paper Seeing Blue, which describes how blue-rich light coincides with the peak sensitive region of the human circadian system and the implications of blue-rich LEDs. It also notes other implications of blue light, such as increased glare for the aging eye and greater skyglow from increased scatter.
Image: The circadian system (dashed line) is most sensitive at the wavelength of blue-rich LED (blue peak), about 450 nanometers; courtesy of International Dark-Sky Association.
Buy the Right Light Bulb
So what does the Nobel angle have to do with screwing in a light bulb? If you are going to change your outdoor lights, choose a soft white LED bulb that does not impinge on the entire animal kingdom, including humans!
Light bulbs have a rating system that indirectly announce which ones are better for your health and the environment. While the initial wave of LED lights were blue-intensive (and Nobel-worthy), manufacturers have since moved beyond that easy-to-make color and offer better options. The lower the Color Correlated Temperature (CCT), the redder the light; the higher the CCT the bluer the light. The rating system is usually printed on the packaging--you just have to look for it.
The US Dept. of Energy has a Lighting Facts guide for label reference. Personally I think the guide is overly generous in describing "warm" as up to 3500K, so I choose 3000K as an easy reference number. In practice, for outdoor lights, lower is better.
A Matter of Aesthetics
Moving beyond the science of what makes a good outdoor light bulb choice, consider the aesthetics. Are we really trying to turn night into daytime? Isn't the soft white light more pleasing to the eye at night than the garish hue of a blue-rich light? Once you install an LED light--for better or for worse--it will be around for many years, so please choose wisely.
For outdoor use, do not buy blue-rich LEDs that are rated above 3000K (such as those Daylight or Cool bulbs). Instead, buy LEDs that are rated 3000K or below, which are often dubbed Soft or Warm. A desirable and commonly available option is the LED rated at 2700K. If I were a Nobel Prize winner, that's what I'd do.
Then again, if you don't like the Nobel-minded option for outdoor bulb selection, there's the Minion option instead...