There's a little tool called f.lux that claims:

During the day, computer screens look good — they're designed to look like the sun. But, at 9PM, 10PM, or 3AM, you probably shouldn't be looking at the sun.

F.lux fixes this: it makes the color of your computer's display adapt to the time of day, warm at night and like sunlight during the day.

It's even possible that you're staying up too late because of your computer. You could use f.lux because it makes you sleep better, or you could just use it just because it makes your computer look better.

Is it true that the color temperature of a computer screen could upset one's biorhythm?

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    I use f.lux. So used to it that I can't even tell it's enabled in the evening and at night. If I turn it off, the whiteness is almost blinding :) – Chris Dennett May 21 '11 at 0:23
  • I also just started using it. The premise has some plausibility to it: If the surroundings are dark, a darker screen is certainly more pleasant to look at. But I'm curious about the claim that the sun-light quality of the unchanged screen could trick the body into thinking it's actually daytime... – Lagerbaer May 21 '11 at 0:27
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    I use it. It does make the screen less painful to look at at night, but my sleeping schedule is still f**ked. So take that for what it's worth. – Jesse Aldridge May 21 '11 at 0:39
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    Another f.lux user here. Awesome application. Some more anecdotal evidence that it does work from me. I have retinitis pigmentosa so white (i.e. blue) light is very detrimental to my vision. I don't know if this makes f.lus especially useful to me, but it undeniably has helped my sleeping patterns. – user2466 May 21 '11 at 0:52
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    Bright light therapy can be a useful treatment for people whose circadian rhythms are disordered. There are a collection of links to papers on the bottom of this page for possible leads on this question. I think it's highly plausible that the colour temperature of the screen could upset one's biorhythm. – user2466 May 21 '11 at 0:59

One study says, "possibly". Note that the illumination provided by the light sources used in the study may not be consistent with the light coming from a computer monitor. Also note that the sample size is not large, and a mechanism for the effect is not proposed. (source)

J Physiol Anthropol Appl Human Sci. 2005 Mar;24(2):183-6.

Effect of color temperature of light sources on slow-wave sleep.

Kozaki T, Kitamura S, Higashihara Y, Ishibashi K, Noguchi H, Yasukouchi A.

Department of Physiological Anthropology, Faculty of Design, Kyushu University, Japan. kozaki@design.kyushu-u.ac.jp

In order to examine whether the spectral compositions of light source may affect sleep quality, sleep architecture under different color temperatures of light sources was evaluated. Seven healthy males were exposed to the light sources of different color temperatures (3000 K, 5000 K and 6700 K) for 6.5 h before sleep. The horizontal illuminance level was kept at 1000 lux. Subjects slept on a bed in near darkness (< 10 lux) after extinguishing the light, and polysomnograms recorded the sleep parameters. In the early phase of the sleep period, the amount of stage-4 sleep (S4-sleep) was significantly attenuated under the higher color temperature of 6700 K compared with the lower color temperature of 3000 K. Present findings suggest that light sources with higher color temperatures may affect sleep quality in a view that S4-sleep period is important for sleep quality.

A recent article in Scientific American (source) discussed a possible mechanism for the effect.

Many years later researchers extended Keeler’s observation, showing that mice genetically engineered to lack rods and cones (the light receptors involved in vision) nonetheless reacted to changes in light by adjusting their circadian clock—the internal timer that synchronizes hormone activity, body temperature and sleep. The animals performed the usual daytime activities when in daylight and nighttime activities when in the dark. They could do so even though their retinas lacked the photoreceptor cells that vertebrate eyes use to form images, although surgically removing their eyes abolished this ability. This phenomenon may be common to many mammals, including humans: recent experiments have shown that certain blind people can also adjust their circadian clocks and constrict their pupils in response to light.

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    Nice find. The retina is an important organ in melatonin production. This is quite interesting. – user2466 May 22 '11 at 7:41

Yes, the blue colour may affect sleep cycles, but an app might not be enough to fix it, depending on other lighting conditions.

The existing answers are somewhat theoretical, without considering whether there is a pragmatic difference caused by real computer screens.

There has been a lot of research into the effect of blue light from computer screens and smart-phones and how they affect circadian rhythyms in humans.

Does screen light have any effect at all?

Twelve young adults were put in a dim room for four hours before bedtime, for five nights in a row. Each night, half read a printed book and half read from an eBook which emitted light.

We found that the use of these devices before bedtime prolongs the time it takes to fall asleep, delays the circadian clock, suppresses levels of the sleep-promoting hormone melatonin, reduces the amount and delays the timing of REM sleep, and reduces alertness the following morning. Use of light-emitting devices immediately before bedtime also increases alertness at that time, which may lead users to delay bedtime at home. Overall, we found that the use of portable light-emitting devices immediately before bedtime has biological effects that may perpetuate sleep deficiency and disrupt circadian rhythms, both of which can have adverse impacts on performance, health, and safety.

It is worth noting that a letter in response argued that the effect was confounded by the subjects not being exposed to normal light patterns during the day.

That is, the light emitted from the eReaders would have had a much smaller effect in alerting the brain than they would have had the participants been exposed to a normal pattern of everyday light exposure before using the eReaders before bedtime. Thus, the question still remains as to whether the light being emitted from an eReader, or any other type of electronic device, would actually impact nocturnal alertness and sleep in normally behaving individuals.

However, the original authors responded rejecting that their study had this shortcoming:

However, we disagree with the inference that the lighting conditions in our laboratory study overestimated the real-life effects of reading from a light-emitting eReader compared with reading from a printed book. First, participants in these studies were not “spending the entire day in dim room lighting.” [...] Second, such minor differences in daytime ambient lighting are unlikely to account for our findings.

(If it is easier reading, this study was discussed and put in context in this essay.

Is the Colour-Temperature Relevant?

In this study thirteen participants looked at iPad screens for a couple of hours under different conditions - just the screen (on full brightness), with additional blue leds shining on them, and with orange-tinted goggles that cut out the blue light. They had they melatonin-levels measured.

The present study extends results from Figueiro et al. (2011) showing that a 2-h exposure to self-luminous tablets can result in a measurable, statistically reliable suppression of melatonin in Table 1.

That is, not wearing the orange goggles to cut out the blue light, lead to measurable changes in their melatonin levels, when viewing a regular iPad screen. Adding additional blue light increased the effect.

They add:

it is important to acknowledge that usage of self-luminous electronic devices before sleep may disrupt sleep even if melatonin is not suppressed. Clearly, the tasks themselves may be alerting or stressful stimuli that can lead to sleep disruption. For now, however, it is recommended that these devices be dimmed at night as much as possible in order to minimize melatonin suppression, and that the duration of use be limited prior to bedtimes.

What can be done?

This article takes a relatively theoretical approach, estimating the amount of different types of light that are produced by smartphone displays in typical use, and proposes some approaches to minimise the effects:

At night, if we use smartphone displays that are darker than that of the CIL threshold of light which activates circadian systems, the unhealthy effect of smartphone displays can be minimized.

It still requires further experimental support. However, they were able to highlight the limits of what can be achieved by adjusting the smartphone alone.

If we use smartphones at the proper distance and brightness setting in a dark room, the MSV can drop below ~1%. However, the use of a smartphone in a bright room at night significantly increases the [circadian illuminance] and MSV values [...] If people use smartphones in a bright room at night, it will be a little difficult to decrease the blue effect of smartphones on human health by varying smartphone variables that users can adjust. From the results of tuning the emitting wavelength of blue LEDs in a smartphone LCD backlight, fine control of blue light in smartphone displays can have a greater impact on reducing the unhealthy effect of blue light from smartphone displays at night. Thus, combating the unhealthy effect of blue light from smartphone displays at night does not simply mean turning off the smartphone, but rather using well designed [spectral power distribution] and the proper intensity of smartphone displays to see better where and when required while protecting people’s health and circadian rhythm.

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The color temperature of lighting can be called warm or cool. A cool light has more blue than a warm or neutral light.

Setting your computer monitor or phone to a brighter setting produces more light in general, and a cool setting produces a higher proportion of blue/red light.

The night mode on your phone changes the proportion of blue/red, shifting it toward the red. The dark modes in the OS and apps merely reduce the overall screen brightness.

Wikipedia and many other sources claim that bright light in general affects sleep patterns. But with regard to specific color temperature the Harvard Health website states it best by summarizing some literature on these effects. In these cases the effects of blue light on melatonin are discussed. Melatonin is a hormone that regulates circadian rhythms and has negative health consequences if produced at inappropriate times.

An excerpt from the article

While light of any kind can suppress the secretion of melatonin, blue light at night does so more powerfully. Harvard researchers and their colleagues conducted an experiment comparing the effects of 6.5 hours of exposure to blue light to exposure to green light of comparable brightness. The blue light suppressed melatonin for about twice as long as the green light and shifted circadian rhythms by twice as much.


In another study of blue light, researchers ... compared the melatonin levels of people exposed to bright indoor light who were wearing blue-light–blocking goggles to people exposed to regular dim light without wearing goggles. The fact that the levels of the hormone were about the same in the two groups strengthens the hypothesis that blue light is a potent suppressor of melatonin.

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    "states it best by citing studies"? I would love to see a citation, but I can't see any on that page. In particular, I want to know if the subjects were bathed in a large amount of light, or just whatever a phone can put out. Is this just an extrapolation, or a pragmatic study of phone or computer use? – Oddthinking Apr 3 '19 at 0:54
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    The goal here is to provide definitive evidence. I think it is likely that some studies were done, but I am not at all convinced that they are on the scale of the light from computer screens, as opposed, for example, to light boxes used to treat Seasonal Affective Disorder, which are on the order of 1000 times as bright (in lux) as a computer screen. – Oddthinking Apr 3 '19 at 12:34
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    Skimming this work, it seems to have nothing to do with computers. It is focussed on which part of the eyes are responsible for having their circadian rhythms affected by light. The light did not come from computer screens, but they had subjects kept in special rooms for many days, exposing them only to special light cues. – Oddthinking Apr 3 '19 at 12:47
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    -1: Your answer doesn't address the question, and relies on an unreliable source. We aren't asking for repeats of the claim. We are asking for empirical evidence as to whether the claim is true, and this answer doesn't address that, except via speculation. – Oddthinking Apr 4 '19 at 0:50
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    Originally, it was to help you improve your answer. Now, it seems you refuse, it is to help others vote appropriately. – Oddthinking Apr 4 '19 at 1:02

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