For an experience that commands a quarter to a third of our lives, surprisingly little is known about how our bodies and brains sense the need to sleep, then automatically adjust and discharge that need so that we can function day in and day out.
At the center of it all is the circadian clock, a cluster of cells composing the suprachiasmatic nucleus (SCN) tucked into the brain’s hypothalamus, that oscillates on a schedule that synchronizes our life—and all other life on the planet—with patterns of solar light. Each of us is born with this basic mechanism of predictability, which is built into the brain and reiterated in every cell.
The molecular clock doesn’t only regulate the amount and quality of sleep we get. It has a big hand in overall physical and mental health as well. Body temperature, heart rate, hormone levels, immune response, mood, alertness, cognitive performance, auditory reaction times—all have their moments in the sun, as solar time is biologically etched within us.
Just how sweeping are the hands of the circadian clock? Evidence is amassing that violating the circadian clock undermines mental and metabolic health and sets the stage for neurodegenerative conditions. Desynchronization of biological rhythms is now near ubiquitous in the industrialized world, brought on by mistimed or near-constant exposure to light, most notably specific wavelengths of it, and disrupting functions as diverse as person processing, endocrine response, and antioxidant activity.
The Blue-Light Destabilizer
While the invention of electric light began blurring the boundary between day and night more than a century ago, it took the digital revolution to confound it altogether. The blue light emitted by digital devices turns out to be the strongest force to set—and upset—the circadian clock
Blue light is part of the visible light spectrum; exposure to it in the morning is physically vitalizing and mood-enhancing. It sets the human rhythm, kicking off biological day. Even small amounts of it, studies show, suppress production of melatonin, the circadian-tied hormone that ushers in biological night, building up as light diminishes and promoting sleepiness.
Exposure to blue light any time suppresses melatonin production, but mistimed exposure to it—say, at night—desynchronizes the circadian clock. Seventy-five percent of children and 70 percent of adults use a light-emitting electronic device in bed, according to the Sleep Foundation.
Brain function is especially tied to the timing and type of light exposure because photoreceptors in the human eye—retinal ganglion cells—communicate directly with the brain, sending signals to the SCN master clock. By virtue of a unique constituent protein called melanopsin, photoreceptors at the front of the eye are most sensitive to blue-light wavelengths. Stimulation of melanopsin-containing photoreceptors during the day aligns circadian, neuroendocrine, and neurobehavioral activity with environmental time.
But melanopsin receptors have their own circadian schedule, and they are particularly sensitive during evening and nighttime hours. That’s why exposure to even small amounts of blue light at night can have effects in destabilizing the circadian system.
How Sleep Staves Off Anxiety
Night-shift work notoriously begets circadian confusion, and those who perform it—14 percent of all U.S. workers—are known to be at risk of sleep problems, accidents, impaired glucose tolerance, cardiovascular problems, and breast cancer.
Even one night’s sleep loss can have a substantial effect on the lives of healthy people. It can bias psychological reactions negatively. A team of researchers at Uppsala University reports that one night of sleep loss leads otherwise healthy young adults to negatively interpret facial expressions, seeing people as angrier than they actually are.
In the same vein, scientists at the University of Bern recently discovered that one of the functions of dream sleep is to triage emotions, consolidating the storage of positive ones while dampening consolidation of strongly negative or traumatic ones. It does this by decoupling electrical activity of neural cell bodies and dendrites in the prefrontal cortex, allowing signals of danger to activate dendrites but blocking them from entering cells. In the absence of dream sleep, the researchers find, there’s no discrimination between signals of safety and of danger, and neurons generate a fear reaction, setting the stage for anxiety disorders.
Another function of sleep, Columbia University researchers report, is to relieve oxidative stress, a cause of cell damage to which the brain is highly vulnerable due to its ferocious consumption of oxygen. Lack of sleep, it turns out, increases sensitivity to oxidative stress. Over time, cellular damage accumulates, manifesting in neurodegenerative disease.
As the prime circadian mediator, melatonin is a natural agent of sleep. Studies show that its biological effects extend well beyond sleep to help influence the conditions affected by sleep loss and circadian disruption. It combats oxidative stress. It acts as a neuroprotectant, shielding brain cells from the accumulation of metabolic debris. It moderates glucose metabolism. It suppresses growth of cancer cells.
There are many ways to protect sleep. Prime among them is limiting exposure to blue light in the evening and night. It’s wise to circumscribe screen time and stop computer activity at least two hours before bedtime. While you’re at it, take the TV out of the bedroom.
Taking supplemental melatonin late in the day to bring on biological night is an increasingly popular way to combat insomnia. According to the National Institutes of Health, melatonin use in the U.S. has more than quintupled since the millennium, as exposure to disruptive blue light at night has soared.
Taking melatonin is not risk-free. High doses at night can create sleepiness during the next day. Melatonin is a hormone, and it can interfere with some biological functions. But most of all, studies are inconclusive on how effective it is as a sleeping pill. There’s some evidence that melatonin may be no more helpful than a placebo. Then again, researchers note, placebos are highly effective as sleep-inducing agents.
- Of all the colors in the visible light spectrum, blue light has the strongest effect on circadian rhythms.
- Exposure to the short-wavelength blue light from screens in smartphones and tablets mimics exposure to morning sunlight.
- Age-related changes to the eyes reduce the amount of light reaching the circadian clock, impeding synchronization and disturbing sleep in the elderly.
- The declining mental health of young people may reflect not just the content of social media but the timing of exposure to the blue-light-emitting devices that convey it.
(By Hara Estroff Marano published September 6, 2022 in Psychology Today Magazine.)
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