I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be.
— William "Lord Kelvin" Thomson. Lecture on "Electrical Units of Measurement" (3 May 1883), published in Popular Lectures Vol. I, p. 73
If you have sleep trouble or you're trying to learn more about sleep in general, we recommend understandingsleep.org.
Study: Reading on a tablet vs. book
In 2014, a new study was published in PNAS (full text) that compares the effects of reading an iPad before bed, versus a regular paper book.
In 2012, the American Medical Association's Council on Science and Public Health made this recommendation
"Recognizes that exposure to excessive light at night, including extended use of various
electronic media, can disrupt sleep or exacerbate sleep disorders, especially in children and
adolescents. This effect can be minimized by using dim red lighting in the nighttime
Blue Light Affects Sleep (and here's why)
We know that night-time exposure to blue light keeps people up late. We believe that f.lux adjusts screens in a way that greatly reduces the stimulating effects of blue light at night.
To understand the effects of f.lux on sleep, we've spoken with some researchers, and we've read a whole lot of papers.
The science that explains why blue light keeps you up was begun many years ago in the study of bird migration, and it continued in humans with the discovery of a new photoreceptor in the eye, called Melanopsin. Many are familiar with the "rods and cones" that provide our visual capabilities, but it was only about 15 years ago that retinal ganglion cells containing melanopsin, which are sensitive to a broad range of blue-green light around 480nm, were discovered, and their unique effect on sleep was investigated.
The experimental research suggests that an average person reading on a tablet for a couple hours before bed may find that their sleep is delayed by about an hour. Clearly, the details are complicated, but that's why we get to cite so many very interesting papers.
Our f.luxometer project has data for dozens of devices and filters (available for re-use). The "References" section there is much more up to date, so for a current view of the circadian impact of light, we recommend taking a look.
Popular press coverage of blue light research
In Eyes, a Clock Calibrated by Wavelengths of Light
cites work by Cajochen et al in the May issue of The Journal of Applied Physiology.
Christian Cajochen did the first work we know of that uses real LED displays and measures them against older computer monitors. Many studies before this used light sources that you could believe were "like" these newer displays, but this paper tests the exact kind of backlight that we all use many hours a day.
David C. Holzman does a survey paper on the effects of blue light, with citations as early as 1958.
What's in a Color? The Unique Human Health Effects of Blue Light
Holzman DC 2010. What's in a Color? The Unique Human Health Effects of Blue Light.
Environ Health Perspect 118:A22-A27. doi:10.1289/ehp.118-a22
NIH-funded research is available in its full text online. If you've tried to search for journal articles and read them online, you know that a large number of scientific journals today are difficult to access, except from within an institution that subscribes. The following bibliography includes the NIH-published versions of some of the work done at Brigham and Women's Hospital (Harvard Medical School).
Exposure to Room Light before Bedtime Suppresses Melatonin Onset and Shortens Melatonin Duration in Humans
Joshua J. Gooley, Kyle Chamberlain, Kurt A. Smith, Sat Bir S. Khalsa, Shantha M. W. Rajaratnam, Eliza Van Reen, Jamie M. Zeitzer, Charles A. Czeisler, Steven W. Lockley
J Clin Endocrinol Metab. 2011 March; 96(3): E463E472. Published online 2010 December 30. doi: 10.1210/jc.2010-2098
The human circadian system adapts to prior photic history
Anne-Marie Chang, Frank A J L Scheer, Charles A Czeisler
J Physiol. 2011 March 1; 589(Pt 5): 10951102. Published online 2011 January 10. doi: 10.1113/jphysiol.2010.201194
High sensitivity of the human circadian melatonin rhythm to resetting by short wavelength light.
Lockley SW, Brainard GC, Czeisler CA.
J Clin Endocrinol Metab. 2003 Sep;88(9):4502-5.
PMID: 12970330 [PubMed - indexed for MEDLINE]
Effect of Light on Human Circadian Physiology
Jeanne F. Duffy, Charles A. Czeisler. Sleep Med Clin. Author manuscript; available in PMC 2010 June 1.
Published in final edited form as: Sleep Med Clin. 2009 June; 4(2): 165177. doi: 10.1016/j.jsmc.2009.01.004
Suppression of melatonin secretion in some blind patients by exposure to bright light.
Czeisler CA, Shanahan TL, Klerman EB, Martens H, Brotman DJ, Emens JS, Klein T, Rizzo JF 3rd.
N Engl J Med. 1995 Jan 5;332(1):6-11.
PMID: 7990870 [PubMed - indexed for MEDLINE]
A phase response curve to single bright light pulses in human subjects
Sat Bir S Khalsa, Megan E Jewett, Christian Cajochen, Charles A Czeisler
J Physiol. 2003 June 15; 549(Pt 3): 945952. Published online 2003 April 25. doi: 10.1113/jphysiol.2003.040477
Amplitude Reduction and Phase Shifts of Melatonin, Cortisol and Other Circadian Rhythms after a Gradual Advance of Sleep and Light Exposure in Humans
Derk-Jan Dijk, Jeanne F. Duffy, Edward J. Silva, Theresa L. Shanahan, Diane B. Boivin, Charles A. Czeisler
PLoS One. 2012; 7(2): e30037. Published online 2012 February 17. doi: 10.1371/journal.pone.0030037
Sex difference in the near-24-hour intrinsic period of the human circadian timing system
Jeanne F. Duffy, Sean W. Cain, Anne-Marie Chang, Andrew J. K. Phillips, Mirjam Y. Mόnch, Claude Gronfier, James K. Wyatt, Derk-Jan Dijk, Kenneth P. Wright, Jr., Charles A. Czeisler
Proc Natl Acad Sci U S A. 2011 September 13; 108(Supplement_3): 1560215608. Published online 2011 May 2. doi: 10.1073/pnas.1010666108
Uncovering Residual Effects of Chronic Sleep Loss on Human Performance
Daniel A. Cohen, Wei Wang, James K. Wyatt, Richard E. Kronauer, Derk-Jan Dijk, Charles A. Czeisler, Elizabeth B. Klerman
Sci Transl Med. Author manuscript; available in PMC 2010 July 13. Published in final edited form as: Sci Transl Med. 2010 January 13; 2(14): 14ra3. doi: 10.1126/scitranslmed.3000458
The Impact of Sleep Timing and Bright Light Exposure on Attentional Impairment during Night Work
Nayantara Santhi, Daniel Aeschbach, Todd S. Horowitz, Charles A. Czeisler
J Biol Rhythms. Author manuscript; available in PMC 2009 August 1.
Published in final edited form as: J Biol Rhythms. 2008 August; 23(4): 341352. doi: 10.1177/0748730408319863
Short-Wavelength Light Sensitivity of Circadian, Pupillary, and Visual Awareness in Humans Lacking an Outer Retina
Farhan H. Zaidi, Joseph T. Hull, Stuart N. Peirson, Katharina Wulff, Daniel Aeschbach, Joshua J. Gooley, George C. Brainard, Kevin Gregory-Evans, Joseph F. Rizzo, III, Charles A. Czeisler, Russell G. Foster, Merrick J. Moseley, Steven W. Lockley.
Curr Biol. 2007 December 18; 17(24): 21222128. doi: 10.1016/j.cub.2007.11.034
Entrainment of the human circadian pacemaker to longer-than-24-h days
Claude Gronfier, Kenneth P. Wright, Jr., Richard E. Kronauer, Charles A. Czeisler
Proc Natl Acad Sci U S A. 2007 May 22; 104(21): 90819086. Published online 2007 May 14. doi: 10.1073/pnas.0702835104
Decreased sensitivity to phase-delaying effects of moderate intensity light in older subjects
Jeanne F. Duffy, Jamie M. Zeitzer, Charles A. Czeisler
Neurobiol Aging. Author manuscript; available in PMC 2008 May 1.
Published in final edited form as: Neurobiol Aging. 2007 May; 28(5): 799807. Published online 2006 April 18. doi: 10.1016/j.neurobiolaging.2006.03.005
Efficacy of a single sequence of intermittent bright light pulses for delaying circadian phase in humans
Claude Gronfier, Kenneth P. Wright, Richard E. Kronauer, Megan E. Jewett, Charles A. Czeisler
Am J Physiol Endocrinol Metab. Author manuscript; available in PMC 2009 October 14.
Published in final edited form as: Am J Physiol Endocrinol Metab. 2004 July; 287(1): E174E181. Published online 2004 March 23. doi: 10.1152/ajpendo.00385.2003
Intrinsic period and light intensity determine the phase relationship between melatonin and sleep in humans
Kenneth P. Wright, Claude Gronfier, Jeanne F. Duffy, Charles A. Czeisler
J Biol Rhythms. Author manuscript; available in PMC 2009 July 27.
Published in final edited form as: J Biol Rhythms. 2005 April; 20(2): 168177. doi: 10.1177/0748730404274265
Sensitivity of the human circadian pacemaker to nocturnal light: melatonin phase resetting and suppression
Jamie M Zeitzer, Derk-Jan Dijk, Richard E Kronauer, Emery N Brown, Charles A Czeisler
J Physiol. 2000 August 1; 526(Pt 3): 695702. doi: 10.1111/j.1469-7793.2000.00695.x
Intrinsic near-24-h pacemaker period determines limits of circadian entrainment to a weak synchronizer in humans
Kenneth P. Wright, Jr., Rod J Hughes, Richard E. Kronauer, Derk-Jan Dijk, Charles A. Czeisler
Proc Natl Acad Sci U S A. 2001 November 20; 98(24): 1402714032. doi: 10.1073/pnas.201530198
Phase-shifting human circadian rhythms: influence of sleep timing, social contact and light exposure
J F Duffy, R E Kronauer, C A Czeisler
J Physiol. 1996 August 15; 495(Pt 1): 289297.
Temporal dynamics of late-night photic stimulation of the human circadian timing system
Zeitzer JM, Khalsa SB, Boivin DB, Duffy JF, Shanahan TL, Kronauer RE, Czeisler CA.
Am J Physiol Regul Integr Comp Physiol. 2005 Sep;289(3):R839-44. Epub 2005 May 12.
PMID: 15890792 [PubMed - indexed for MEDLINE]
Adaptation of human pineal melatonin suppression by recent photic history
Smith KA, Schoen MW, Czeisler CA.
J Clin Endocrinol Metab. 2004 Jul;89(7):3610-4. Erratum in: J Clin Endocrinol Metab. 2005 Mar;90(3):1370.
PMID: 15240654 [PubMed - indexed for MEDLINE]
Dynamic resetting of the human circadian pacemaker by intermittent bright light.
Rimmer DW, Boivin DB, Shanahan TL, Kronauer RE, Duffy JF, Czeisler CA.
Am J Physiol Regul Integr Comp Physiol. 2000 Nov;279(5):R1574-9.
PMID: 11049838 [PubMed - indexed for MEDLINE]
Human circadian pacemaker is sensitive to light throughout subjective day without evidence of transients.
Jewett ME, Rimmer DW, Duffy JF, Klerman EB, Kronauer RE, Czeisler CA.
Am J Physiol. 1997 Nov;273(5 Pt 2):R1800-9.
PMID: 9374826 [PubMed - indexed for MEDLINE]
Exposure to bright light and darkness to treat physiologic maladaptation to night work.
Czeisler CA, Johnson MP, Duffy JF, Brown EN, Ronda JM, Kronauer RE.
N Engl J Med. 1990 May 3;322(18):1253-9.
PMID: 2325721 [PubMed - indexed for MEDLINE]
Mental activation and sleep
Some EEG studies of color effect on the brain have been done. One of the earlier ones we found is cited here.
From Effect of Illuminance and Color Temperature on Lowering of Physiological Activity, Noguchi H, Sakaguchi T.
"...we surmise that the effect of color temperature is greater than that of illuminance in an
ordinary residential bedroom or similar environment where a lowering of physiological activity is desirable, and we therefore find the use of low color temperature illumination
more important than the reduction of illuminance. Subjective drowsiness results also indicate
that reduction of illuminance without reduction of color temperature should be avoided."
These results suggest that low color temperature light creates a smooth lowering of central nervous system activity, and that low color temperature illumination can be used effectively in a bedroom or other such environment where it is desirable to facilitate lowered physiological activity.
From Effect of Color Temperature of Light Sources on Slow-wave Sleep", Tomoaki Kozaki , Shingo Kitamura , Yuichi Higashihara , Keita Ishibashi , Hiroki Noguchi  and Akira Yasukouchi 
1. Department of Physiological Anthropology, Faculty of Design, Kyushu University
2. Department of Ergonomics, Kyushu Institute of Design
3. Matsushita Electric Works, Ltd.
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.
Empirical performance studies
There is tons of research on human performance based on sleep, and some interesting research based on mental activation (and productivity) when cooler light colors are used in the work environment.
"The effect of high correlated colour temperature office lighting on employee wellbeing and work performance", Peter R Mills (1,2) , Susannah C Tomkins (1) and Luc JM Schlangen (3)
1. Vielife Ltd, 68 Lombard Street, London EC3V 9LJ, UK
2. Department of Respiratory Medicine, The Whittington Hospital, London N19 5NF, UK
3. Philips Lighting, Global Organisation Applications Lighting, P.O. Box 80020, 5600JM Eindhoven, The Netherlands
Journal of Circadian Rhythms 2007, 5:2doi:10.1186/1740-3391-5-2
The amount of blue light in the spectrum of light sources increases with increasing colour temperature. So far a number of studies have investigated the effects of the colour temperature of lighting on mental activity, the central nervous system and alertness. These studies have demonstrated that higher colour temperatures (7500 K versus 3000 K) are more activating from the viewpoint of mental activity level . Both the parasympathetic and sympathetic nervous systems are thought to be enhanced under higher colour temperature conditions.  and drowsiness has been observed to be higher under lower colour temperature lighting when comparing 3000 K with 5000 K .
"Effects of indoor lighting (illuminance and spectral distribution) on the performance of cognitive tasks and interpersonal behaviors: The potential mediating role of positive affect"
In Study 2, subjects exposed to warm white light reported stronger preferences for resolving interpersonal conflicts through collaboration and weaker preferences for resolving conflicts through avoidance than subjects exposed to cool-white light. Additionally, illuminance and spectral distribution (color) interacted to influence subjects' self-set goals on a clerical coding task. In Study 3, receipt of a small, unexpected gift and exposure to warm-white light both increased the amount of time subjects were willing to donate as unpaid volunteers. In addition, in the absence of a gift, subjects volunteered more time under low than under high illuminance.
General blue light notes
Terman and Terman report in "Light Therapy for Seasonal and Nonseasonal Depression: Efficacy, Protocol, Safety, and Side Effects" in CNS Spectrums:
Recent attention has focused on the blue region, which actively suppresses melatonin production (23) and elicits circadian rhythm phase shifts. (24, 25) In a comparison of blue light with red light of lower intensity (designed as a placebo control), the antidepressant response to blue was superior, similar to that seen for white light in other studies. (26)
23. Brainard GC, Hanifin JP, Greeson JM, et al. Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor. J Neurosci. 2001;21:6405-6412.
24. Wright HR, Lack LC, Kennaway DJ. Differential effects of light wavelength in phase advancing the melatonin rhythm. J Pineal Res. 2004;36:140-144.
25. Warman VL, Dijk DJ, Warman GR, Arendt J, Skene DJ. Phase advancing human circadian rhythms with short wavelength light. Neurosci Lett. 2003;342:37-40.