The Science of Circadian Rhythms

By Brandon Peters, MD

Circadian rhythms may be the most difficult concept to understand in the world of sleep medicine. There is a lot of confusing language, and it relies on science that is not easily approached. Fortunately, a basic understanding of the science of circadian rhythms can be obtained and may help to explain some cases of insomnia and daytime sleepiness.

As the World Turns

The Earth’s 23 hour and 56 minute daily rotation provides predictable rhythms of light and temperature, food, and predator activity. Through adaptive evolution, our body’s metabolism and even our behaviors are programmed to respond to this precise timing. Franz Halberg coined the term circadian (from the Latin meaning “about a day”) in 1959. It describes numerous approximately 24-hour cycles that are generated within nearly every organism on the planet.

Within our body, there is a system in place that measures time and synchronizes many internal processes to daily events within the environment. Some of these important processes include:

• Sleep and wakefulness

• Metabolism

• Core body temperature

• Cortisol levels

• Melatonin levels

• Other hormones (growth hormone, thyroid hormone, etc.)

The control of these patterns is built into our genetic makeup; the machinery synchronizes rhythms that will persist independently of outside influences. The first mammalian gene, Clock, was identified in 1994. Multiple additional genes have been identified that constitute a core molecular clock that gives rise to other cellular, tissue, and organ function.

Every cell in our body follows a circadian pattern: an extraordinary symphony of biochemical reactions that are perfectly timed based on available resources and orchestrated by a small group of cells in the anterior part of the brain’s hypothalamus. Through hormones and other as yet undetermined influences, the central pacemaker coordinates peripheral clocks that are present in cells as diverse as cardiac, liver, and adipose tissues.

Light is perceived by the eyes and travels via the retina to the optic nerves. Above the optic chiasm, where the two optic nerves cross behind the eyes, sits the suprachiasmatic nucleus (SCN). This is the master clock of the body. It couples the numerous physiological processes described to the timing of light and darkness in the environment.

These patterns will persist without external time cues, but they may vary slightly from the geological day length. As a result, in isolation from resetting cues, the timing of these processes may gradually become desynchronized. The degree of shift may depend on our genetic program, or tau, with most people having an internal clock that runs longer than 24 hours. It is understood that our genetics and the interaction with other environmental factors – especially the exposure to morning sunlight – may have important effects on resetting the internal clock. These external influences are called zeitgebers, from the German for “time-givers”.

When the internal clock is misaligned to our environment, circadian disorders such as delayed and advanced sleep phase syndrome can occur. With a complete disconnect from light perception, as occurs in total blindness, a Non-24 rhythm occurs. These conditions are often associated with insomnia and excessive daytime sleepiness as well as irregularities in the sleep-wake rhythm that cause social and occupational dysfunction.

Fortunately, treatment of circadian disorders can be highly effective and a board-certified sleep physician can provide helpful guidance and resources.

Sources:

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Czeisler, CA et al. “Bright light resets the human circadian pacemaker independent of the timing of the sleep-wake cycle.” Science. 1986;233:667-671.

Lewy, AJ et al. “Phase shifting the human circadian clock using melatonin.” Behav Brain Res. 1996;73:131-134.

Moore RY and Eichler, VB. “Loss of a circadian adrenal corticosterone rhythm following suprachiasmatic lesions in the rat.” Brain Res. 1972 Jul 13;42(1):201-206.

Moore-Ede, MC et al. “A physiological system measuring time, “ in The Clocks That Time Us. Cambridge, Massachusetts, Harvard University Press, 1984, p. 3.

Peters, BR. “Irregular Bedtimes and Awakenings.” Evaluation of Sleep Complaints. Sleep Med Clinic. 9(2014)481-489.

Piggins, HD. “Human clock genes.” Ann Med. 2002;34(5)394-400.

Reid, KJ and Zee, PC. “Circadian disorders of the sleep-wake cycle,” in Principles and Practices of Sleep Medicine. Edited by Kryger MH, Roth T, Dement WC. St. Louis, Missouri, Elsevier Saunders, 2011, pp. 470-482.

Richardson, G and Malin, HV. “Circadian rhythm sleep disorders: pathophysiology and treatment.” J Clin Neurophysiol. 1996;13:17-31.

Sack, RL and Lewy, AJ. “Circadian rhythm sleep disorders: lessons from the blind.” Sleep Medicine Reviews. 2001;5(3):189-206.

Vitaterna, MH et al. “Mutagenesis and mapping of a mouse gene, Clock, essential for circadian behavior.” Science. 1994;264(5159):719-725.

How Does Screen Light Affect Sleep?

By Brandon Peters, MD

In a world increasingly dependent on technology, bright screens are more commonly part of our everyday life. These screens range broadly in size and purpose: televisions, computers, tablets, smartphones, e-books, and even wearable tech. How does this artificial light, especially when viewed at night, potentially impact our sleep? Learn how light at night affects our body’s circadian rhythm and whether it might contribute to insomnia and difficulty awakening. In addition, consider ways to reduce light exposure and counteract its effects.

Before Thomas Edison and His Light Bulb

It is hard to imagine a time before artificial light existed. It is such an integral part of our lives that we consider ourselves deeply inconvenienced when we lose power in a storm. Think back to what life was like before modern technology like computers and televisions, before light bulbs, and even before electricity.

Primitive societies and people were highly dependent on the natural availability of light. The sun ruled life. It is no surprise that it was worshiped in ancient Egypt. When artificial light became possible, things dramatically changed.

The Influence of Light on the Body’s Functions

All life on Earth has developed patterns of activity dependent on the timing of day and night. When isolated from the natural environment, innate circadian patterns will be revealed. As an example, most humans have an internal clock that runs just over 24 hours in length. However, light profoundly affects the timing of sleep and wakefulness, metabolism, and hormone release.

Morning sunlight has a key influence on life’s functions. It promotes wakefulness and ends sleep. It can help shift the desire for sleep slightly earlier. In the winter, when sunlight comes later, we may want to sleep in or suffer from symptoms called winter depression.

Due to the length of our internal clock, our bodies have a natural tendency towards delay in the timing of our sleep. This means that we always find it easier to go to bed and wake later. Have you ever noticed how easy it is to stay up another 15 minutes but how difficult it is to wake just 15 minutes earlier? Morning sunlight can profoundly reset this internal clock.

How Artificial Light at Night Impacts Sleep

Unfortunately, artificial light at night can negatively affect the timing of our sleep. Light shifts sleep timing, and light at night can shift our desire for sleep later. This can result in difficult falling asleep, as occurs with insomnia. Night owls, or those with delayed sleep phase syndrome, may be especially susceptible.

Not everyone is sensitive to these effects. If you are especially sleepy, perhaps due to inadequate total sleep time or poor sleep quality, you are unlikely to be impacted.

There are several important factors to consider:

The Source of Light

Artificial light can come from light bulbs and many other sources, including televisions, computers, tablets, smartphones, e-books, and even wearable tech. Each of these can generate a different intensity of light. Near screens may have more impact that those across a room.

The Amount of Light

Most overhead lights generate a light intensity that varies from about 500 to several thousand lux. For comparison, full sunlight at midday may be 100,000 lux in intensity. Commercially available lightboxes often generate about 10,000 lux. The screen of your smartphone may create hundreds of lux of light, depending on the settings you use. Even smaller amounts of light, such as from a tablet screen, may have an impact in some people.

The Color of Light

Much is made of the fact that blue light is responsible for shifting circadian rhythms. Full spectrum light – what you might consider as “white light” or “natural light” – contains the blue wavelengths. Blue-blocker sunglasses (with an amber or orange lens) and screen filters are sold to block this light wavelength.

The Timing of Light

One of the most important variables is when you are exposed to light, including from artificial sources. There is evidence that light at night could shift your desire for sleep by about 1 hour. This delays your ability to fall asleep and may impact your desire to wake in the morning.

Therefore, it is very important to turn down the lights at night, especially preceding your bedtime. Some people may need to avoid excessive artificial light exposure for the 1-2 hours before going to bed. This means turning off the phone, powering down the computer, and avoiding light from tablets, e-books, and other sources.

Instead, try to stick to low-tech options: listen to some music, read a book printed on old-fashioned paper, or page through a magazine. By reducing and eliminating your exposure to light at night, you may find that you are able to sleep better. If you continue to struggle, speak with a sleep doctor about additional treatment options.