Why is sleep important?
Oxygen, water, food, sleep. These are the four requirements for life. Without them (in varying lengths of time) people die. Sleep is an important factor as it heavily influences our performance at work, our risk for chronic disease, and our overall quality of life. Sleep researchers, however, aren’t clear on the exact reason why humans even need to sleep. From an evolutionary perspective sleep would certainly have left primitive humans vulnerable to attack so it must offer some great advantages. Generally speaking, sleep is thought to allow our brains and our bodies to heal and repair from the damage of the day; synaptic plasticity is managed, brain pathways that are not being used are pruned, muscles are repaired, and energy stores are replenished.
America is Tired
Short sleep duration (defined as less than 7 hours of sleep per night) is common. In the United States 24% – 48% of people report it. If you are obese, a smoker, consume excessive alcohol, or are physically inactive you are more likely to report short sleep duration.1 Sleep is an active process. In other words, you don’t fall asleep simply because your brain is tired. Sleep is actively maintained throughout the night. Disruption of this careful balance between arousal and sleep results in, what else, disrupted sleep.
Circadian Rhythm is Important
Humans display a 24-hour circadian rhythm. This rhythm is endogenous meaning it is maintained even in the absence of environmental cues. Human beings placed environments with no light, no time cues, no social cues, etc., will still display a 24-hour rhythm. However, external timing cues do modulate and adapt the rhythm to the environment. For example, sunlight, in humans, is a powerful cue to have the waking phase of our 24-hour cycle during the day. Other mammals, like rats and mice, have most of their waking phase at night. These rhythms were created through many years of evolutionary pressure. Light cues, which modulate our circadian rhythm, are extremely powerful. Circadian rhythm is endogenous so an internal pacemaker is necessary. The suprachiasmatic nucleus in the hypothalamus is a major part of the internal pacemaker of humans. This part of the brain is connected to the retina via the retinohypothalamic tract.2 This direct and powerful connection shows just how important light is for controlling circadian rhythm.
Stages of Sleep
Sleep is divided into two forms – non-REM and REM. Non-REM sleep is further subdivided into 4 stages, each with its own properties. During non-REM sleep neuronal activity is low, metabolic rate and brain temperature are at their lowest, heart rate decreases, blood pressure lowers, and muscle tone and reflexes are intact. Contrast this with REM (rapid eye movement) sleep where brain activity is similar to that of the awake brain. For this reason, it is sometimes referred to as paradoxical sleep. In REM sleep brain temperature and metabolic rate rise and there is complete loss of muscle tone with the exception of the diaphragm (so you can breathe), the eye muscles, and some muscles within the inner ear.
The Adverse Effects of Blue Light
A recent study of about 1,500 Americans found that 90% of adults use an electronic device within 1 hour of bedtime at least a few times per week. Devices may be an important contributor to insomnia because of the short-wavelength-enriched light that is emitted from them. Exposure to light in the evening and the early part of the night, even at low intensity, has several sleep-disturbing effects. First is suppresses melatonin delaying the onset of sleep. It shifts the circadian clock to a later time making it harder to fall asleep at a regular hour. Finally, it increases alertness and arousal.3
In one interesting study, 12 adults were randomly assigned reading from a light-emitting eBook for 4 hours before bed or reading from a print book for 4 hours before bed for 5 consecutive evenings. They found that those reading an LE-ebook displayed decreased subjective sleepiness, decreased EEG delta/theta activity, suppressed melatonin secretion, lengthened sleep latency (increased time to fall asleep), delayed and reduced REM sleep, and impaired morning alertness.3
Why does blue light have this effect? Human eyes have rods, cones, and intrinsically photosensitive retinal ganglion cells. Rods and cones are mostly responsible for image-forming vision. Intrinsically photosensitive retinal ganglion cells are responsible for regulating circadian rhythms and other biological functions. Retinal ganglion cells form the beginning of the retinohypothalamic tract which transmits light data from the retina to the hypothalamus to regulate circadian rhythms throughout the body. Retinal ganglion cells respond powerfully short-wavelength light like the blue-colored light emitted from most devices and LED televisions. This blue light will cause a high rate of fire from the eyes to the hypothalamus, which will change hormonal and circadian rhythms.4 Blue light also has the ability to stimulate other areas of the brain which are responsible for producing norepinephrine, a neurotransmitter that has potent arousal properties.5
With the knowledge that the blue light emitted from almost all devices and LED TVs is disruptive to the foundational pathways for sleep, it’s important to take steps to remedy this if you’re having trouble sleeping. First and foremost, do not use your device in bed. At all. In fact, you should avoid using it at least an hour before you plan to go to bed. Do not watch TV in bed either. If your symptoms are severe, avoid TV for at least an hour before you plan to retire. If you have to use a device for some reason, avoid long exposure. You should also consider wearing red-colored lenses for about an hour before bed. The red-color of the lens blocks the blue light from reaching the retina. Blue-light blockers that are clear also exist. You could consider purchasing a pair of these and using them in the evening and while you’re using your device. I’ve recommended a pair from this website: www.blepeyewear.com. As devices age, they tend to emit more blue light. If possible, consider getting a newer device. Finally, many devices come with a night-shift mode. This mode shifts the hue of the screen to a warmer, redder tone. This has been studied to see if it offset the negative effects of the light from a device. Unfortunately, it did not. Melatonin levels were still reduced when using night shift mode.6
In the end, if you suffer from insomnia, reducing your device exposure is likely to have a measurable effect on your sleep. Our devices have become extensions of our lives so it may be difficult to completely eliminate exposure, however, your brain (and your boss) will thank you for the better sleep.
2Kandel, et al., editors. “Sleep and Dreaming .” Principles of Neural Science, 4th ed., McGraw-Hill, 2000, pp. 936–947.
3Proc Natl Acad Sci U S A. 2015 Jan 27;112(4):1232-7.
4Molecular Vision 2016; 22:61-72.
5Blue-Light Therapy following Mild Traumatic Brain Injury: Effects on White Matter Water Diffusion in the Brain. Front. Neurol. 8:616.
6Sleep, Volume 40, Issue suppl_1, 28 April 2017, Pages A290.