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How Light and Sunsets Affect Circadian Rhythms and Sleep

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Your circadian rhythms control many parts of your life, including your cycles of sleeping and waking. Regulated by your brain's suprachiasmatic nucleus, your circadian rhythms help keep your body in balance and functional. However, many factors, including ones out of your control, can disturb that balance — such as the seasonal changes that make sunset arrive a little bit earlier each day.

What Are Circadian Rhythms?

Circadian rhythms are the natural processes that regulate our bodies in relationship to the cycle of the 24-hour day. Many people think of their circadian rhythms as their internal clock. Circadian rhythms have an effect on many of the body's systems and functions, including digestion, but they're most involved with the sleep-wake cycle.

Before the invention of artificial light, circadian rhythms were determined by the rising and setting of the sun. People got up around sunrise, and they went to bed shortly after sunset, even as the seasons changed.

Artificial light, however, has changed that equation. Because electric light allows people to conduct daytime activities after dark, many people's circadian rhythms and sleep schedules are thrown off. Long workdays, night shifts, eating late at night and even just staying up to binge-watch favorite TV shows into the wee hours disrupt the natural sleep-wake cycle once determined by the sun.

As a result, many people in modern society suffer from poor sleep. Their sleep deprivation can have drastic effects on their quality of life overall. Misaligned circadian rhythms and lack of sleep have been linked to depression, cognitive impairment, obesity, heart disease, diabetes and mood disorders. Sleepiness also, of course, can lead to accidents of all sorts. In many ways, disturbances of the sleep-wake process leave many people living in a state of what feels like permanent jet lag.

How Circadian Rhythms Work

The suprachiasmatic nucleus in the brain stays busy 24/7. While it's in charge, it coordinates cells throughout the body, in essence, turning them on and off in sync with the body's overall circadian rhythm.

Located in the hypothalamus, the suprachiasmatic nucleus responds to light and dark. When light travels from the optic nerve to the suprachiasmatic nucleus, it sends out signals throughout the brain, triggering the release of certain hormones and raising or lowering body temperature.

Among other activities, the suprachiasmatic nucleus controls the release of melatonin each night to let the body know it's time to go to sleep, and it sends out the hormone cortisol in the morning to help the body wake. Research into these functions has made great leaps over the past two decades, with the explanation of the sleep-wake mechanism winning a Nobel Prize in 2017.

Thanks to the work of the suprachiasmatic nucleus, people's circadian rhythms rise and fall throughout the day. The drive for sleep is at its strongest between 2:00 a.m. and 4:00 a.m. in most people, with a second dip in the circadian rhythm occurring around 1:00 p.m. to 3:00 p.m. — just in time for an afternoon nap.

However, teenagers often experience a disruption to this standard adult rhythm. For them, the sleep phase of the circadian cycle is pushed back a few hours, so their bodies aren't ready for sleep till about 11:00 p.m., with their greatest circadian dip occurring between 3:00 a.m. and 7:00 a.m. or even later — which, for many teens, is the time they have to show up for school. (If you have a teen at home, this explains why it's so hard to get them up in the morning.)

Exacerbating the problems with getting up in the morning — not just for teens but for adults as well, is the presence of artificial light. When people are exposed to plenty of light long after the sun has gone down, the body's natural circadian rhythms can be disrupted. Another change that affects circadian rhythms: the impact of the loss of daylight that comes every fall as the days get shorter and the sun sets earlier.

The Effect of Seasonal Changes and Earlier Sunsets on Circadian Rhythms and Sleep

As the sun starts to set earlier and earlier with winter approaching, people's bodies become further misaligned, with biological time becoming less and less in sync with what some scientists call "social time." Because the changes seem imperceptible since sunset only differs by a minute or two each day, people don't suffer from the disorientation caused by jet lag — but the effects are present, nonetheless.

Sunlight continues to act as a cue for the suprachiasmatic nucleus — but the fact that sunrise is at 7:20 a.m. doesn't matter if the alarm clock is going off at 6:00 a.m. In the same way, the earlier sunsets of fall and winter start to trigger the suprachiasmatic nucleus to prepare the body for sleep. But if the sun is disappearing at 4:30 in the afternoon, it only means more electric lights are being turned on, further disrupting the circadian process.

Studies have shown that, despite modern dependence on artificial light, people can still sync their circadian rhythms with the natural rise and setting of the sun very quickly. In a 2013 study from the University of Colorado Boulder, integrative psychologist Kenneth Wright showed that people who removed themselves from artificial light restored the sync of their natural circadian rhythms within less than one week.

After that time, all the study participants were in sync with the rhythms of the natural sunlight they were exposed to, regardless of how disturbed their circadian rhythms were before the study began. Another encouraging sign is that people stayed in sync with their natural, sun-driven biological rhythms once they returned to their normal lives.

Tips for Adjusting Sleep Patterns to the Change of Seasons

People who struggle with changes to their sleep patterns resulting from the shift in seasons can take a few steps to help weather the adjustment successfully. If you're finding your circadian rhythms and sleep impacted by the loss of daylight (and the need to turn on more artificial light), try these tips:

  • Open the curtains and expose yourself to as much bright, natural light as possible in the morning. This should help you feel more alert and trigger the release of cortisol needed to face the day.
  • Recognize that getting enough sleep is as important to overall health and well-being as working out, and prioritize time for adequate sleep.
  • If you're not exposed to much natural light during the day at work, turn up the lights during morning and early afternoon hours, and try to get outside as much as possible.
  • Turn down the lights beginning at sunset to parallel the loss of natural light outside. Using a simple dimmer switch can make this easy.
  • Step away from screens — TVs, smartphones, tablets and computers — an hour or two before your planned bedtime. The blue-toned artificial light emanating from devices is particularly detrimental to sleep.

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