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How familiar do the following sound?
Learning about your natural sleep-wake cycle, or your circadian rhythm is the first step in understanding the answers to many of these questions.
So what are circadian rhythms? The word 'circadian' comes to us from Latin 'circa' = about & 'dies' = day. It refers to the physical, mental and behavioral changes that follow a roughly 24-hour cycle, responding to light and darkness in an organism's environment. These rhythms are found in most living things, including animals, plants and many tiny microbes [2].
Why 24 hours? Scientists believe that our circadian rhythm evolved over millions of years as a response to the earth's 24-hour rotation period. The earth's rotation causes a predictable pattern of day and night. Being synchronized with the planet has obvious benefits and is crucial to the survival of many organisms.
How is the rhythm set? It is now generally accepted that circadian rhythms occur endogenously (meaning they are built in) however the rhythms are strongly affected by external cues such as light and temperature. Over a short term, the daily rhythm can be influenced by diet, stress, activity levels and other health factors.
What co-ordinates the many clocks within our bodies? Up until now we have referred to circadian rhythm as if it were a single clock - in reality there are many periodic oscillations within the human body. In animals the regulator of the overall circadian rhythm is a region in the brain called the suprachiasmatic nucleus (SCN). You can think of the SCN as the master coordinator, it sends out signals to other regions of the brain and in this way regulates specific functions within the body. A key signal the SCN sends is to the pineal gland to secrete melatonin. Melatonin is also known as the 'vampire hormone' or 'sleep hormone' because it's levels are elevated at night time in the absence of light.
What do these body clocks regulate? Functions regulated by the clock include hormone levels, digestion, immune function, and body temperature. Each keeps track of their own clock while also being calibrated and kept in sync with each other through the SCN.
A visual summary of the rhythm is provided below:
Image Credit: Wikimedia Public Domain
So what if my body clock is out of sync? Ignoring your body's natural rhythm is a bad idea. In December 2007, after a 10 year research study, the World Health Organization famously declared shift work that disrupts circadian rhythms a Class 2A carcinogen [3]. Circadian disruption has been linked to various sleep disorders, such as insomnia. Abnormal circadian rhythms have also been associated with obesity, diabetes, depression, bipolar disorder and seasonal affective disorder.
This article is just a start - but there is a lot more you can learn about your sleep wake cycle.
Although our circadian rhythm is endogenous, it is constantly being entrained (adjusted) by environmental cues. These cues are essential to ensuring our internal clocks line up with the earths 24-hour cycle.
The main agent of circadian adjustment is light. It has been shown through research, that a pulse of light administered before sunrise will shift the 'awake' phase earlier and the same pulse of light administered after dusk will shift delay the 'sleep' phase. Your body is constantly adjusting its sleep-wake cycle to reflect external day and night.
Scientists have been long aware of the primary visual tract: the mechanism of light transfer from the retina, through the retinal ganglion axoms, to the optic nerve and eventually for processing in the visual cortex areas of the brain.
However, there is another mechanism of stimuli transfer that bypasses this system entirely. This is known as the retinohypothalamic visual tract. The SCN which we described earlier, as the master-coordinator receives information directly from the eye from special cells called photosensitive retinal ganglion cells (pRGC's). Although these cells detect light they are not used in sight, instead stimuli from these cells travels along the optic nerve directly to the SCN.
The concept is illustrated in the schematic below.
So what does blue light have to do with all this? The photopigment melanopsin (the molecule that detects light in the cell) inside the pRGCs, is primarily activated by the shorter wavelengths of visual light ranging from violet to cyan, with the most significant activation occurring with blue light (∼420 nm - 480 nm) [4]. The results are still relatively recent and it's likely that more information will be forthcoming as the field evolves.
Why blue light? There is no clear reason why blue light is so important as a circadian entrainment agent. It has been hypothesized that blue light is important because it's the colour of the sky - a light source that all our early ancestors would have been exposed to.
For more information on the unique role of blue light, read this article.
Why is this important? This is important because today we are exposed to more blue light at the wrong times than ever before. All these devices that we love and use - phones, tablets and laptops - emit blue light. Depending on the intensity of the light and some other factors, the blue light emitted activates your PRG's, which in turn tricks your brain into thinking it's day-time.
It's not that blue light is bad - it's blue light after dusk that's the issue. Since most devices are used late into the night, the SCN attempts to entrain the circadian rhythm by delaying the sleep phase.
What do Blue Blocking Glasses do? Blue blocking glasses prevent blue light from reaching the retina. The absence of blue light stimuli on the PRG's mimics natural physiological darkness. This in turn causes the SCN to signal the pineal gland to increase melatonin production. As melatonin production increases we feel sleepier and thus shift our phase sleep earlier.
What does the research show? This study tested the hypothesis that cutting the blue portion of the light spectrum with orange lens glasses (blue blockers) would prevent the light-induced melatonin suppression. They concluded that blue blockers represent an elegant means to prevent the light-induced melatonin suppression.
Another study of the effectiveness of blue blocking glasses conducted in Switzerland shows that wearing blue blocking glasses at night time prevented light-induced melatonin suppression and alerting effects in young adults, and significantly improved subjective sleep quality after a continuous 2-week application.
Although the results are promising we should note that further research is underway. More importantly, larger studies testing more criteria should be conducted to fully evaluate the ideal conditions under which blue light blocking glasses are effective.
There are numerous articles online, read the following to start understanding how light, melatonin and sleep are all interlinked:
Notes:
[1] Consult your medical professional for a clinical diagnosis or before making any changes to your daily habits.
[2] As defined by the National Institute of General Medical Sciences
[4] Bailes, H. & Lucas R., The Royal Society. Human melanopsin forms a pigment maximally sensitive to blue light (λmax ≈ 479 nm) supporting activation of Gq/11 and Gi/o signaling cascades. Another study by Brown & Robinson indicates peak sensitivity lies between 420 nm & 440 nm.