It seems so odd - almost counterintuitive - at first. The idea that light could be harmful to our health seems crazy! But there is an abundance of evidence showing that light at night, especially blue light from our phones and laptops, is increasing the rates of a bunch of chronic diseases.

Let me give you a basic overview of the science, and then I'll go into the research studies that show the impacts.

Almost all living creatures - from cyanobacteria to plants to animals - have built-in circadian rhythms that govern their biology over a 24-hour cycle.  We humans are no exception! The quick examples that come to mind include your inborn sleep/wake cycle, your body temperature fluctuation over the course of the day, and your hormones, such as cortisol, that rise and fall rhythmically.

Scientists have discovered over the past couple of decades that the mechanism for re-setting your circadian clocks is light. Specifically, blue light in the ~480nm wavelength hitting the special receptors in your eyes signals that it is daytime.

Melatonin, often thought of as the sleep hormone, begins to rise each evening as daylight dwindles. Sunlight (or bright light) hitting your eyes in the morning stops the production of melatonin for the day.  You probably see where I'm going with this...

We now have lots of artificial light at night. Mr. Edison invented the light bulb over 100 years ago, and as a society, we haven't looked back.  The advent of color TV pouring blue light into our living rooms every night began a new era. Now, almost everyone is exposed to blue light at night from cell phones, laptops, tablets, TV's – and CFL/LED overhead lights.   All of this is in stark contrast to all of human history with only the warm glow of firelight, with very little light in the blue spectrum, at night.

I was initially extremely skeptical that light at night was all that important.  So what if we are going to bed a little later due to watching TV while looking at Facebook on our phones!?!  

I started a document to just keep track of studies that I came across that related to circadian rhythm, light at night, chronic disease, etc. When that document hit about 20 pages in length, I realized that this stuff is important. Really important! Why isn't everyone talking about it important! Gotta actually do something important!

Let me present some of the research on the topic so you can decide for yourself. Whenever possible, I've chosen sources that are open access so that you can read the whole journal article instead of just the abstract.

1.) Melatonin is suppressed by light in the blue wavelengths - between 470-480 nm.[ref]  This has been well known as far back as 1985, although a lot of the genetic components weren't elucidated until more recently.[ref]

2) Melatonin is more than just a hormone to make you sleepy! It plays a bunch of important roles in the body, and having less of it when you are supposed to (at night) affects so many systems of the body.  

  • It is a signaling molecule to set our circadian clock.
  • It is an antioxidant.[ref]
  • It is neuroprotective.[ref]
  • It is anti-inflammatory.[ref]

3) Breast Cancer. (This was a big eye opener for me!)

  • Light at night and decreased melatonin play a big role in breast cancer. There are a lot of really good, solid studies on this. One well-respected study explains that that light at night "induced disruption of the circadian nocturnal melatonin signal promotes the growth, metabolism, and signaling of human breast cancer and drives breast tumors to endocrine and chemotherapeutic resistance".[ref]
  • Studies looking at light at night vs. cancer rate (even taking into account social and economic confounders) mostly show an increased risk of breast cancer due to light at night. A Connecticut study shows 63% increase for those in the higher quarter of light exposure compared to the lowest quartile.[ref] A Georgia study, on the other hand, only showed a 12% increase in breast cancer due to light at night.[ref] A study of teachers in California found a 34% increase in the risk of breast cancer for premenopausal women exposed to higher amounts of light at night.[ref]
  • Experimental models using human breast cancer cells grafted into rats show that light at night markedly increases the growth of the cancer cells.[ref]
  • Studies of shift workers have repeatedly shown an increased risk for breast cancer.  An analysis that combined a bunch of studies found that each 10-year increment of shift work increased the relative risk of breast cancer by 16%.
  • Melatonin actually has been shown to modulate the effect of BPA on causing breast cancer.[ref]

4) Mood disorders. (16% of adult women are now on antidepressants.)

  • A study of young adult psychiatric patients found that low melatonin levels correlated with the highest depressive symptoms scores.[ref]  
  • Several neurotransmitters vary according to a circadian rhythm. Increasing light at night disrupts these neurotransmitter levels.[ref][ref]
  • Genetic variants in the core circadian clock genes have been repeatedly linked to increased susceptibility to anxiety, depression, and bipolar disorder. Looking at the genes that increase risk of a condition is an excellent way to determine the mechanism causing the condition.[ref][ref][ref][ref][ref]
  • Mouse studies show how light at night is affecting our core circadian genes. Exposing mice to dim light at night prior to adolescence increases adult anxiety-like behaviors.[ref] And mice exposed to 'aberrant light' conditions exhibit depression, even when they still get enough sleep.[ref]

5) Obesity. (Another public health epidemic on the rise over the past few decades.)

  • Light at night has been linked in a number of studies to increased risk of obesity.[ref][ref][ref]
  • The mechanism for this is possibly due to melatonins role in insulin sensitivity. Basically, people are more insulin sensitive in the morning when melatonin is suppressed and more insulin resistant at night when melatonin rises.[ref]
  • The hunger hormone, ghrelin, and satiation signal, leptin, are also tied to light and melatonin.[ref]

6) Alzheimer's disease.

  • Recent studies are finding that there is a bi-directional relationship between circadian disruption and Alzheimer's disease. This implies that circadian disruption is a cause of Alzheimer's disease – as well as Alzheimer's pathology leading to worsening circadian disruption.[ref]
  • A study of young adults found that sleep deprivation (not sleeping for the entire night) increase beta-amyloid by 5% after just one night.[ref]
  • Mouse studies allow us to see that the accumulation of amyloid-beta plaque begins years before Alzheimer's symptoms occur. These studies also show us that disturbing circadian rhythm can cause an accumulation of amyloid-beta plaque.[ref]
  • Animal studies also show that exposure to dim light at night causes neurodegeneration in animal models of Alzheimer's.[ref] And an animal model of APOE E4 (genotype that increases risk of Alzheimer's) shows directly that dim light cause circadian disruption and increased tau protein deposits in the brain.

7) And more...  Other disorders linked to light at night or circadian disruption include diabetes[ref], cardiovascular disease[ref], and insomnia[ref].