Greg Yeutter
Dec 7, 2018 · 6 min read
Photo by Dayne Topkin on Unsplash

Perhaps you’ve heard the news: blue light messes with sleep. But few people, even among those in the lighting community, really understand the scientific links between blue light and sleep.

In this article, I’m going to break down those barriers. Grab your prisms, because you’ll see for yourself why both blue and green light alert the circadian rhythm, and why the overall design of a light source makes a huge difference.

Looking at spectra

Today, we are going to compare a bunch of color spectral power distributions, which we’ll call spectra for short. Each type of light has a unique spectral signature. Let’s first get a basic understanding of how these work.

This is the normalized spectrum of a common halogen light:

Halogen Light

A few things to note about the graphic above:

  • The colors on the plot correspond to real-world visible colors
  • The numbers at the bottom of each plot are the corresponding wavelengths in nanometers
  • The height of each color is the relative intensity of that color. So, for the halogen light, there is relatively less blue than red
  • The human visual system is sensitive to approximately 380 to 700 nm
  • The gray area between about 410 and 570 nm is known as melanopic sensitivity, which peaks around 483 nm. This is the “blue light” that triggers the circadian rhythm
  • The white overlay between 410 and 570 nm is the relative circadian input for the given light source

Notice that colors within the melanopic sensitivity zone are not just blue. You’ll also see purple and green. All three can trick the circadian rhythm into thinking it’s daytime.

While it’s uncommon for artificial light sources to produce a lot of purple, they do tend to produce significant blue and green.

Daytime vs. nighttime light

Let’s look at the normalized spectrum of daylight. It’s easy to see that all colors are rendered at a high intensity, and the melanopic sensitivity curve is nearly full:


Now, let’s compare this with the normalized spectrum of nighttime outdoor light (i.e. moonlight):

Nighttime Light

Whoa! It looks like moonlight has nearly the same circadian input as daylight! And even more blue light!

Not so fast…

Daytime, Nighttime, and Indoor Recommended Light Levels

The separate daytime and nighttime spectra I showed were normalized, meaning the actual intensity of the light source was not considered. It turns out daylight is very bright, at about 10,000 lux. Moonlight is extremely dim, at about 0.32 lux.

Even though moonlight has relatively more blue light than red light, when plotted alongside daylight, it is clear that the circadian input is far lower.

Indoor light sources

On the last plot, I showed daylight, nighttime light, and indoor light. Although both nighttime light and indoor light are a lot dimmer than daylight, let’s zoom in on the indoor light:

Nighttime and Indoor Recommended Light Levels

These light sources are both much dimmer than daylight, but there is a huge difference between them. As stated before, outdoor light is extremely dim, at about 0.32 lux. Meanwhile, the recommended indoor task lighting level is 500 lux. The “Indoor Recommended” light source is halogen in this instance.

While 500 lux is nowhere near the brightness of daylight, it is plenty bright to disrupt the circadian rhythm at night. When the body expects <1 lux, it is getting way more blue and green light. This is what experts warn about regarding nighttime blue light exposure.

Now, 1 lux is way too dim for any practical activities, such as reading and dining. But 500 lux is actually quite intense at night. So much so, that you may experience eyestrain at this brightness. The recommend level really works better in the daytime.

Striking a balance

If 1 lux is too dim, but the recommended task light level provides too much of a nighttime circadian input, what do we do? Strike a balance.

Bedtime Bulb was designed to provide about 100 lux at a usable distance. When you install Bedtime Bulb in a reading lamp, for example, you should get about 100 lux where you typically place a book.

Bedtime Bulb provides about 100 lux at a typical distance

It turns out that, for most people, 100 lux is plenty bright for nighttime activities. Not too dim that you would want something brighter, but not bright enough to cause eyestrain. In addition, Bedtime Bulb provides very low circadian input while allowing you to carry out normal activities.

Here’s how much circadian input Bedtime Bulb provides, compared to the recommended light level:

Nighttime, Indoor Recommended, and Bedtime Bulb Light Levels

Comparing Bedtime Bulb with common light sources at the same distance, you get significantly less circadian input (and eyestrain):

Bedtime Bulb vs. Common Light Sources

Even compared to other low-blue light sources, Bedtime Bulb still provides less circadian input, less eyestrain, and an overall more-comfortable lighting experience:

Comparing Low-Blue Light Sources

We ran the numbers on a number of light sources, and we found Bedtime Bulb had the lowest circadian input (melanopic lumens) of all, even among low-blue sources. At the same time, the numbers show that it provides a very comfortable (350 lm, 2200 K), very high-quality (97 CRI) light that is plenty bright for nighttime activities:

Bedtime Bulb has the lowest circadian input, very high color quality, and the most comfortable evening light

Seeing the light

In this article, you learned:

  • The spectral signatures of different types of light
  • Why green and purple light provide circadian input, in addition to blue
  • Why indoor light can disrupt the circadian rhythm, even though it is much dimmer than daylight

The data clearly show that Bedtime Bulb is the ideal light source for use in the hours leading up to bed. But data are one thing, and real world experiences are another. Hundreds of customers have told us Bedtime Bulb is the perfect evening light.

If you liked this post, you may also be interested in my recent interview on the Health by Design podcast. We cover lighting spectra and a whole lot more.

To learn more about Bedtime Bulb, visit our website (Canada version).

About the author

Continuing a lifelong obsession with light, Greg Yeutter founded SimpleBulb Inc. to make lighting simple and healthy. Bedtime Bulb is the beginning of the #healthylight revolution.

Some spectral power distribution data: f.luxometer. The rest were captured by Greg using a Konica-Minolta CL-500A illuminance spectrophotometer.


SimpleBulb Inc. makes light healthy. Our product, Bedtime Bulb, reduces sleep-interrupting blue light.

Greg Yeutter

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Founder of Bedtime Bulb: Other stuff:


SimpleBulb Inc. makes light healthy. Our product, Bedtime Bulb, reduces sleep-interrupting blue light.

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