Hello, everyone. Do you enjoy soaking up the sun? In modern times, we often have a negative impression of sunlight, as UV rays cause skin damage. While that may be true from a dermatological perspective, sunlight plays a crucial role in promoting “happiness.” Today, I would like to introduce the multifaceted mechanisms behind this, based on various research findings.
1. Sunlight Stimulates Serotonin Synthesis
Serotonin is perhaps the most famous “happiness hormone,” known for providing a sense of calm and contentment. A study by Lambert et al. (The Lancet, 2002 [Ref 1]) demonstrated that exposure to sunlight increases serotonin levels in the body. By conducting blood tests to directly measure the rate of serotonin production, the researchers evaluated its relationship with meteorological conditions. The results showed a direct positive correlation between serotonin levels and the duration of sunshine on any given day, with levels rising rapidly as light intensity increased. This study is highly significant because it provides direct evidence by measuring actual serotonin amounts in the blood.
2. Light Stimulates Brain Regions Governing Emotion via the Retina
You may have heard that a disrupted daily rhythm—such as a reversed day-night cycle—leads to emotional instability. It was previously thought that this occurred primarily through the Suprachiasmatic Nucleus (SCN), the center of the internal body clock. However, recent research suggests a more direct path.
A study by Fernandez et al. (Cell, 2018 [Ref 2]) revealed that light travels through a specific neural circuit: ipRGC → PHb → vmPFC. This means there is a direct transmission of neural signals to the emotional centers of the brain via a pathway separate from the SCN. In their experiment, mice exposed to a normal light cycle (T24) showed high scores in sucrose preference tests (an indicator of pleasure), while those exposed to an abnormal cycle (T7) scored significantly lower. This highlights the importance of a regular light cycle for emotional stability.
3. UV Exposure Triggers β-Endorphin Secretion
β-endorphin belongs to a group of substances called endogenous opioids, which induce feelings of euphoria. The mechanism behind this euphoria was clarified by Johnson et al. through experiments on rat cells (The Journal of Neuroscience, 1992 [Ref 3]). In a brain region called the Ventral Tegmental Area (VTA), opioids do not excite dopamine neurons directly. Instead, they inhibit the activity of “interneurons” that release GABA—a substance that normally suppresses dopamine release. By inhibiting the inhibitor, opioids indirectly promote dopamine release, resulting in euphoria.
While β-endorphins are commonly associated with “runner’s high,” what happens when we are exposed to UV rays? Fell et al. (Cell, 2014 [Ref 4]) investigated β-endorphin secretion and opioid-related behavior in mice exposed to UV light. When mice were exposed to UVB (equivalent to 20–30 minutes of Florida sun, five times a week), their blood β-endorphin levels rose significantly over six weeks. This resulted in an increased tolerance to mechanical stimuli and heat—a classic analgesic (pain-relieving) effect of opioids. This effect disappeared when an opioid antagonist was administered.
The pathway for this synthesis was identified in earlier research (Cui et al., Cell, 2007 [Ref 5]): UV irradiation of keratinocytes (skin cells) → Synthesis of POMC protein → Production of α-MSH (the hormone causing tanning) + β-endorphin.
Conclusion
As we have seen, various research results suggest that light provides us with a sense of happiness through multiple biological pathways. While we must remain mindful of the risks of skin cancer, I feel inspired to pursue a happier life by embracing a regular rhythm of light exposure. After all, it just feels good to be outside, doesn’t it?
References
【Reference１】G.W.Lambert et al., Effect of sunlight and season on serotonin turnover in the brain. THE LANCET(2002). DOI: 10.1016/S0140-6736(02)11737-5
【Reference２】D.C.Fernandez et al., Light Affects Mood and Learning through Distinct Retina-Brain Pathways. Cell(2018). DOI: 10.1016/j.cell.2018.08.004
【Reference３】S.W.Johnson and R.A.North., Opioids Excite Dopamine Neurons by Hyperpolarization of Local Interneurons. The Journal of Neuroscience(1992). https://doi.org/10.1523/JNEUROSCI.12-02-00483.1992
【Reference４】G.L.Fell et al., Skin β-Endorphin Mediates Addiction to UV Light. Cell(2014). DOI: 10.1016/j.cell.2014.04.032
【Reference５】R.Cui et al., Central Role of p53 in the Suntan Response and Pathologic Hyperpigmentation. Cell(2007). DOI: 10.1016/j.cell.2006.12.045
Glossary of Terms
PHb (Perihabenular nucleus): A cluster of neurons located in the epithalamus, specifically surrounding the habenular nucleus in the deep brain. It plays a key role in relaying light signals to emotional centers.
SCN (Suprachiasmatic Nucleus): Located in the hypothalamus, the SCN acts as the “master clock” of the body, coordinating peripheral biological clocks found in cells throughout the entire system.
ipRGC (Intrinsically Photosensitive Retinal Ganglion Cells): The “third type” of photoreceptor in the retina (distinct from rods and cones). These cells are responsible for transmitting non-visual light information, such as brightness and day-night cycles, to the brain.
vmPFC (Ventromedial Prefrontal Cortex): A critical hub within the prefrontal cortex involved in high-level mental activities, including emotional regulation and decision-making.
T24: A normal light-dark cycle consisting of 12 hours of light followed by 12 hours of darkness.
T7: An abnormal, ultra-short light cycle consisting of 3.5 hours of light followed by 3.5 hours of darkness, used in research to study the effects of disrupted circadian rhythms.
β-Endorphin: A type of peptide hormone belonging to the opioid family. It is naturally produced by the body to alleviate pain and induce feelings of well-being.
Endogenous Opioids: A group of substances produced within the body (such as β-endorphins) that bind to “opioid receptors” to produce analgesic (pain-killing) effects and euphoria.
VTA (Ventral Tegmental Area): A small but vital region in the midbrain that serves as a central hub for motivation, pleasure, and the brain’s reward system.
GABA (γ-Aminobutyric Acid): An inhibitory neurotransmitter that acts as a “brake” in the nervous system. Normally, GABA suppresses the release of dopamine; however, when β-endorphins are present, they stop GABA’s activity, “releasing the brake” and allowing dopamine to flow freely, which creates euphoria.
Interneurons: Relay neurons that sit between other neurons in the brain and spinal cord, acting as intermediaries to process and organize the flow of information.
Keratinocytes: The primary type of cell found in the epidermis (the outermost layer of the skin), making up about 90% of its structure.
POMC (Pro-opiomelanocortin): A precursor polypeptide made of amino acids. It is broken down by the body to create several different hormones, including β-endorphins and α-MSH.
α-MSH (Alpha-Melanocyte-Stimulating Hormone): A hormone responsible for tanning. It acts on melanocytes in the skin to stimulate the synthesis of melanin.
