In this article, we'll be taking a quick break from our napping series to discuss the topic of circadian rhythms, which we touched on briefly in the previous article.

Introduction

Our 24-hour internal clock or circadian rhythm is heavily influenced by light exposure. This is because the circadian cycle is not exactly 24 hours. It requires daily synchronization by light. This synchronization is imperative for optimal health because the circadian rhythm determines which genes are active and inactive during the day. If gene expression is mistimed, disorders and metabolic disease become more likely. 

When the sun goes down, melatonin production is increased. Melatonin makes you feel sleepy by decreasing your core temperature. Figure 1 shows how melatonin is released according to daylight conditions. Melatonin production begins at sundown and ends at sunrise. 

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Figure 1: Sunset, depicted by the black moon and stars, starts the production of melatonin which brings your core temperature down and makes you sleepy. Sunrise has the opposite effect, decreasing melatonin in preparation for you to wake. (adapted from Flo-Groeneboom 2013)

Since the absence of light begins melatonin production and the presence of light stops melatonin production, it is possible to alter the timing of your circadian rhythm by altering your light exposure. Increasing blue light can be used to decrease sleepiness while blocking light from your sleep environment can help you fall asleep faster and help you to stay asleep. The type of light that you are exposed to is also important. Warm lights that have wavelengths in the red to yellow part of the spectrum (like the sky at sunset) can signal melatonin production to begin. White or blue light signals daytime to your brain and will help to inhibit sleepiness (Figure 2).

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Figure 2: Warm lights towards the red end of the spectrum mimic sunset and help stimulate melatonin production. Blue lights and bright white light mimic daytime sunlight and suppress melatonin and sleepiness. (Suni 2023)

Studies show that circadian rhythms shift due to light and are not sensitive to altering sleep schedules alone. Melatonin is suppressed even when subjects are only exposed to brief flashes of light, while asleep with their eyes closed. It’s imperative to keep light out of your sleeping environment until you want to wake up to avoid disrupting your circadian rhythm.

Findings around light exposure and circadian rhythm shifts:

  • Blue light exposure causes the biggest delays in melatonin.
  • Blue light (from electronics) delays melatonin production but said production recovers quickly, just 15 minutes after exposure.
  • Morning blue light significantly decreases melatonin and reduces sleepiness.
  • Exposure to any light while sleeping - regardless of whether your eyes are closed - significantly decreases sleep quality. Removing any light-emitting electronics from the bedroom can help increase sleep quality.
  • Light pollution affects sleep quality which can be reduced by black out shades and other light-reducing window coverings.

When is our circadian rhythm established?

Babies establish a circadian rhythm after birth based on light exposure. Before a baby is born, its circadian rhythm is determined by its mother. The mother’s brain sends melatonin to the fetus through the placenta. Once a baby is born, it develops its own circadian rhythm within the first month of independence from the placenta. Temperature fluctuations in line with Figure 1 appear at one month of age, while sleep patterns brought on by light exposure begin at six weeks of age. Premature newborns show a wide range of timing around the development of a circadian rhythm; it is likely that brain maturation plays a role in circadian rhythm development along with environmental light. Misalignment of gene expression in the baby can occur if there is a disruption in circadian rhythm via actions of the mother or other environmental conditions post-birth.

Why is the proper alignment of circadian rhythm important in fetuses & babies? 

  • It can alter the proper development of organs causing miscarriage or death
  • Long term sleep disorders can develop in newborns.
  • Susceptibility to infections increases (gene expression errors impair immune function).
  • Cancer can arise later in life from impaired immune function (via circadian disruptions).
  • Those born prematurely or who have mothers with sleep disorders are more likely to develop metabolic syndrome as adults.

Light therapy has proven effective to establish circadian rhythm in the neonatal intensive care unit (NICU). Babies who tune in their circadian rhythm to match regular daylight hours eat more and gain more weight (Figure 3). 

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Figure 3: Preterm infants in the NICU were split into two groups. The control group (yellow circles) was exposed to normal NICU lighting over 24 hours. The experimental group (black circles) was exposed to darkness between 7pm and 7am and lights were on from 7am to 7pm. Infants exposed to normal daylight hours ate more, gained more weight, and spent less time in the NICU before being cleared to go home. (Sánchez-Sánchez 2022)

Pregnant mothers with circadian disruptions may also use light therapy as a possible fix during pregnancy. 

Is light therapy for adults effective?

For those who work the night shift or travel across time zones, light therapy can help reduce sleepiness, mitigate brain fog, and decrease reaction time. Work related accidents are highest in frequency in the last few hours of night shifts. Morning traffic accidents have their highest frequency among drivers who just completed the night shift.       

In Figure 4, light therapy for 30 minute given near end of a night shift at 5am to see if it can help mitigate sleep deprivation in adults working a night shift. Light therapy from both light-emitting glasses and light boxes was found to increase vigilance, cognition, and a positive mood during the early mornings of night shift workers. 

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Figure 4: The vertical blue bar represents the time spent using light therapy. Compared to the control group that had no light therapy, sleepiness was reduced (top graph), reaction time decreased (bottom left), and cognition was improved (bottom right). (Comtet 2019)

In addition to establishing a healthy circadian rhythm (A.K.A. one that properly matches gene expression), light therapy has also had success in treating depression and seasonal affective disorder (SAD). Side effects from light therapy can include visual discomfort and headaches. Light therapy should be administered for thirty minutes and at sunrise or shortly after.

What commercial products are available for light therapy?

  • Light boxes should be used in a stationary space and should have:
  1. A light level of at least 10,000 lux.
  2. A surface area is at least 200 square inches
  3. Emission of white or blue light – not UV which can damage the eyes
  • Light therapy glasses (allows wearer to be mobile during treatment):
  1. Takes only 30 minutes to emit enough circadian rhythm stimulating light
  2. Consider the comfort and weight of glasses if you use during early morning exercise

Getting outside in the sunlight during the morning and day can also reduce daytime sleepiness and help to correct disruptions in the circadian rhythm. In conclusion, mood and vitamin D production are not the only reasons that being outside are important to health! Your circadian rhythm depends on daylight exposure to keep your body systems functioning appropriately during the correct time of day.

References

Antiosek, D. (2024, October 20). Scientific Review of the Best SAD Light Therapy Glasses of 2024. Retrieved from optimizeyourbiology.com: https://optimizeyourbiology.com/best-light-therapy-glasses

Comtet, H., Geoffroy, P. A., Frisk, M. K., Hubbard, J., Robin-Choteau, L., Calvel, L., . . . Bourgin, P. (2019). Light therapy with boxes or glasses to counteract effects of acute sleep deprivation. Nature: Scientific Reports, 9(18073).

Flo-Groeneboom, E. (2013). Sleep and Health in Shift Working Nurses. University of Bergen. researchgate.

Pruitt, J. (2024, May 20). After Testing the Top Options, We Found the Best Light Therapy Lamps for a Mood Boost. Retrieved from health.com: https://www.health.com/condition/depression/best-light-therapy-boxes?utm_source=googlepaid&utm_medium=con&utm_content=Cj0KCQjwveK4BhD4ARIsAKy6pMJz00CV2Qb2GUW0ErmUi01NCeJtG9PmJobJkP4837gffvBtEO3HW7EaAiqxEALw_wcB&utm_campaign=commerce-dd-AllArticles_Health_

Quick Hits: New Study Literally Shines Light on Circadian Rhythm Alignment. (2014). Retrieved from Quittingsitting.com: https://quittingsitting.com/new-study-literally-shines-light-on-circadian-rhythm-alignment/

Sánchez-Sánchez, M., García, T. L., Heredia, D., Reséndiz, I., Cruz, L., Santiago, J., . . . Angeles-Castellanos, M. (2022, October 20). Effect of a light-darkness cycle on the body weight gain of preterm infants admitted to the neonatal intensive care unit. Nature: Scientific Reports.

Suni, E., & Singh, A. (2023, November 8). Light and Sleep. Retrieved from Sleepfoundation.org: https://www.sleepfoundation.org/bedroom-environment/light-and-sleep

Tähkämö, L., Partonen, T., & Pesonen, A.-K. (2019). Systematic review of light exposure impact on human circadian rhythm. Chronobiology International, 36(2), 151-170.

Van Gilst, D., Puchkina, A. V., Roelants, J. A., Kervezee, L., Dudink, J., Reiss, I. K., . . . Chaves, I. (2023, August 21). Effects of the neonatal intensive care environment on circadian health and development of preterm infants. Frontiers in Physiology, 14(1243162).