Light: A Happy Pill for Dark Days?

Light: A Happy Pill for Dark Days?

Have you ever had a day where you feel exceptionally good? As in take on the world kind of good? You feel so much better than the previous couple of days that you stop to wonder why.

Then it dawns on you.

The sun is out. It’s been cloudy for the past week but now—SUNSHINE.

You go out to lunch or for a walk just to take in those rays. Sure, it feels warmer than your darkened office space, but it’s the light rather than warmth that’s making a difference.

You purposely don’t wear sunglasses and it feels like the light is coming in through your eyes and massaging that part of your brain that is your happy zone. Are you imagining it or is the sun really affecting how you feel?

In a study reported in the September 2018 issue of Cell we learn that this is not a figment of your or my imagination (1). There is, in fact, a type of retinal cell that transports sunlight directly to the part of our brains that affects mood.

Eyes and the Body’s Master Clock

Circadian rhythms are innate time-keeping functions found in all multicellular organisms. This subject of the 2017 Nobel prize in Physiology or Medicine, circadian rhythms are fueled by daily light-dark cycles and are critical to the function of neurologic, immune, musculoskeletal and cardiac tissues (2). Nearly every mammalian cell is affected by circadian rhythms.

The human body has a circadian master clock, the suprachiasmatic nucleus or SCN. The SCN is a highly innervated tissue located in the hypothalamus (see image). It is connected directly to the retina by the optic nerve, and thus is influenced by external light and dark.

Light enters eyes and is transmitted to SCN and PHb.
Light enters the eyes and affects the SCN (physiologic effects), and as discussed in recent research, Fernandez et al. here, the perihabenular nucleus (behavioral effects). (Image in public domain.)

The retina of the eye is the light gathering instrument for this organ. Historically, it’s been understood that the retina is composed of two cell types, rods and cones, that function in transmitting light and images to the optic nerve, which sends those signals to the brain.

Drawing of the retina with rods and cones, some nervous tissues.
Some parts of the retina. Light enters the eye (from left) and passes through to the rods and cones. Here a chemical change converts the light to nerve signals. Image based on drawing by Ramón y Cajal, 1911 and licensed under Wikimedia commons.

Studies by Hattar et al. in the early 2000s identified that another cell found in the retina, the melanopsin-containing intrinsically photoactive retinal ganglion cells (ipRGCs) as the transmitter of circadian light signals (3). Through this direct connection to the SCN, the circadian master clock, the ipRGCs can influence a wide range of light-dependent functions independent of image processing (4).

Now Fernandez et al. have identified multiple types of ipRGCs. They showed that ipRGCs that mediate the effects of light on learning work via the SCN, while the pathway for light influencing emotions is different.

They discovered a new target of ipRGC cells, the perihabenular nucleus (PHb). The PHb is a newly recognized thalamic region of the brain. The authors showed that the connection between light and mood is regulated by ipRGCs through the PHb versus the SCN. They show that the PHb is integrated into other mood-regulating centers of the thalamic region.

You can see the details of their studies here.

In Conclusion

Daylight, and lack thereof, does affect both our mood and our ability to learn. In this 2018 report, we have learned that the pathways for these effects are distinct, and gain an understanding of a new thalamic region by which the light and mood actions occur. This information could influence development of better drugs and/or therapies for major depressive disorders.

For those of us with seasonal affective disorder, the evidence is undeniable—lack of light can cause issues, from sleep-wake problems, to mood and learning issues.

And while we can’t create sunshine, a special lamp or light box may help to gain some full spectrum light. To learn more about how to choose such a lamp and when to use it, see this Mayo clinic article for details.

References

  1. Fernandez, D.C. et al. (2018) Light affects mood and learning through distinct retinal pathways. Cell 175, 71–84.
  2. Ledford, H. and Callaway, E. (2017) Circadian clock scoops Nobel prize. Nature 550, 18.
  3. Hattar, S. et al. (2002) Melanopsin-containing retinal ganglion cells: architecture, projections, and intrinsic photosensitivity. Science 295, 1065–70.
  4. Hattar, S. et al. (2003) Melanopsin and rod-cone photoreceptive systems account for all major accessory visual functions in mice. Nature 424(6944)76–81.

Tick, Tock! The Molecular Basis of Biological Clocks

A long time ago, before the rise of humans, before the first single celled organisms, before the planet even accumulated atmospheric oxygen, Earth was already turning, creating a 24-hour day-night cycle. It’s no surprise, then, that most living things reflect this cycle in their behavior. Certain plants close their leaves at night, others bloom exclusively at certain times of day. Roosters cock-a-doodle-doo every morning, and I’m drowsy by 9:00 pm every night. These behaviors roughly align with the daylight cycles, but internally they are governed by a set of highly conserved molecular circadian rhythms.

Jeffrey Hall, Michael Rosbash and Michael Young were awarded the 2017 Nobel Prize in Physiology/Medicine for their discoveries relating to molecular circadian rhythms. The official statement from the Nobel Committee reads, “…this year’s Nobel laureates isolated a gene that controls the normal daily biological rhythm. They showed that this gene encodes a protein that accumulates in the cell during the night, and is then degraded during the day. [They exposed] the mechanism governing the self-sustaining clockwork inside the cell.” What, then, does this self-sustaining clockwork look like? And how does it affect our daily lives (1)?

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Molecular Connections Between Sleep Deprivation and Inflammation

Anyone who has travelled across time zones knows how unpleasant it is when the regular rhythm of your biological clock is disrupted. Jetlag results when the body’s internal clock, or circadian rhythm is out of sync with external cues for “day and “night”, resulting in insomnia, extreme tiredness, difficulty concentrating and various other unpleasant symptoms.

On the bright side, jetlag is at least a temporary misery that is usually over after a few days of acclimation to the new time zone. Long-term disruption of the natural sleep/wake cycle, such as encountered by frequent long-distance travellers, shift workers, or people with physiological conditions that affect circadian rhythms, can be much more debilitating. Longer term health effects that have been associated with constant disruption of circadian rhythms include, insomnia, concentration problems, and increased susceptibility to diseases associated with chronic inflammation such as cancer, diabetes and cardiovascular disease.

Despite the fact that many of the genes and proteins involved in central control of circadian rhythms are known, the reason for the implied association between circadian clock components and immune function is not understood. Recently, a paper was published in the July issue of PNAS that identified a potential link between a circadian clock component and chronic inflammation. Continue reading