Get more sun, it really can make you happier!

How long has it been since you had a good sun tan?

A worker's day starts with getting up in the morning, washing up, and rushing out to work. Then he works indoors all day and rushes back home in the evening. After finally making it to the weekend, he can't help but stay up all night, and has to catch up on sleep during the day. Going out is a luxury, let alone sunbathing.

But seriously, families, stop what you are doing and enjoy some time in the sun. The benefits are more than you think!

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Get more sun

There are more benefits than you think

When talking about the benefits of sunbathing, many people's first reaction would be "it can supplement calcium" and "prevent myopia". In fact, the sun is also nature's remote control, regulating our sleep cycle and mood.

1

Sunbathing can make us more clear-headed

Sunlight affects the level of melatonin in our bodies. Melatonin is a hormone that regulates our biological clock and is secreted by the pineal gland in the brain. Melatonin is also nicknamed the "vampire hormone" because it increases in the evening, causing us to feel sleepy and fall asleep, while bright light during the day suppresses its secretion, keeping people alert.

2

Sunbathing can help us sleep well

Sunlight that suppresses melatonin secretion during the day can actually help people sleep better at night. This is because the body needs an important raw material for synthesizing melatonin - serotonin.

Serotonin, also known as 5-hydroxytryptamine, is a hormone that loves sunlight. When there is sufficient sunlight during the day, the content of serotonin increases, which prepares enough raw materials for the production of melatonin at night, making us energetic during the day and sleeping soundly at night.

However, if we stay in a dim environment for a long time, the serotonin reserves will be insufficient, and melatonin will receive the wrong signal and be secreted prematurely. At night, when melatonin is really needed to maintain sleep, it is not enough. This is why sometimes we feel groggy during the day and have difficulty falling asleep at night.

3

Sunbathing can make us feel positive and happy

When there is plenty of sunlight, the level of serotonin in the human body increases. This is because bright light can inhibit the recycling of serotonin by cells and maintain the serotonin level in the body at a higher level. Serotonin is an emotional management master who constantly transmits positive emotional signals to our bodies, which can bring us willpower, a positive attitude and a sense of happiness.

In addition, abundant light can reduce the level of cortisol, the "stress hormone" in the body, so basking in the sun can really make people feel relaxed and comfortable.

4

Sun exposure can prevent depression

A large study of more than half a million British adults showed that more time spent outdoors in daylight each day was associated with fewer symptoms of depression and lower odds of using antidepressant medication.

In addition, many studies have shown that insufficient light is the main factor causing seasonal depression, and some researchers have even proposed using "light therapy" to relieve winter depression.

5

Sun exposure may prevent Alzheimer's

Sunbathing can make our brains more focused and alert, and can prevent Alzheimer's disease.

A large sample data study covering 362,094 people showed that an average of 1.5 hours of light per day (2 hours in summer and 1 hour in winter) can minimize the risk of developing Alzheimer's disease in the future.

In short, the sunshine not only lights up the world, but also our mood. If you are unhappy, go out and bask in the sun!

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Why sunbathing?

Improve concentration and cognitive abilities?

Bright lighting improves human attention and cognitive abilities. One study showed that people performed better on complex tasks under bright light (1000 lux) than under lower light intensity (200 lux).

Not only at work, a 2020 study published in the Proceedings of the National Academy of Sciences showed that the body enters a higher state of alertness when sleeping in brighter light compared to a dim environment, indicating that bright light promotes alertness.

In fact, compared with sunlight, the intensity of indoor artificial light is very low. Even on a cloudy day, the outdoor light is much stronger (sunny day: 100,000 lux; cloudy day: 10,000 lux; indoors: 500 lux), so the light we are exposed to indoors is far from enough to meet our body's needs.

There are many hypotheses in academia about why bright sunlight can promote alertness and cognitive ability, such as the belief that strong light activates the norepinephrine-dopamine system.

Norepinephrine is a hormone that is closely related to brain mechanisms such as attention, concentration, wakefulness, judgment, working memory, and pain, and can turn the brain into a high-speed "working brain." Dopamine is a hormone that brings happiness and rewards, and can turn the brain into a "learning brain" that actively improves itself.

Under the combined action of these two hormones, the brain will urge itself to stay focused, learn faster, and remember more.

Another study published in Cell also provides us with new ideas. Scientists found that if mice with their backs exposed were exposed to 50 millijoules of ultraviolet light per square centimeter (equivalent to humans sunbathing on the beach for 30 minutes), the mice's memory would be better and their athletic ability would be stronger.

The reason why sunbathing makes mice smarter is that sunlight exposure promotes neurons to produce more glutamate, which is the most important excitatory neurotransmitter in the brain and a key molecule for learning and memory.

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People, born towards the light

Perhaps because we have become accustomed to the existence of "light", we often overlook its significance to life.

But our bodies never forget to follow the light, because in our eyes, there is a group of cells that were born solely for sensing light - intrinsically photosensitive retinal ganglion cells (ipRGC).

There are three types of photoreceptor cells on the human retina: rods, cones, and ipRGCs. Rods and cones have the ability to sense light, color, and perform "visual phototransduction", so they are also called photoreceptors. However, ipRGCs do not have visual functions. They are sensitive to light because they express a unique visual pigment, melanopsin.

The cause of myopia is directly related to the photosensitivity of ipRGC. When staying indoors for a long time, the ipRGC receives insufficient light, which will cause a large amount of substantia nigra to be desensitized. This overexpression of substantia nigra will lengthen the eye axis and change the corneal curvature, causing myopia. This is why it is recommended that adolescents in their developmental period must get more sun exposure to meet the photosensitivity needs of ipRGC.

In addition to photosensitivity, ipRGC plays a more important role as a signaler in our body. It can send the light signals it senses to the brain in a timely manner, thereby regulating physiological activities such as circadian rhythm, pupil response to light, and neuroendocrine.

In 2019, scientists at the Salk Institute discovered that there are three different ipRGC cells that allow the brain to distinguish between day and night and maintain the rhythm of the biological clock. This discovery also explains why some blind people can maintain a good sleep-wake cycle, because they may lack cones or rods, but still have ipRGCs responsible for light perception. However, because ipRGCs are very sensitive to short wavelengths such as blue light, playing with mobile phones for a long time at night will make ipRGCs mistakenly think that it is dawn, and then send signals to the brain to inhibit the secretion of melatonin, affecting sleep quality.

The ipRGCs can also regulate body temperature and metabolism based on the intensity of light they sense. A study published in Cell reported that light inhibits brown fat thermogenesis and blood sugar regulation through ipRGCs, thereby saving energy during warm daytime hours.

However, this study also reminds us that if the normal light cycle is disrupted and we are always exposed to strong artificial light at night, we may experience blood sugar disorders and even metabolic diseases.

The sadness at sunset is also related to ipRGC . The journal Nature Neuroscience reported a special neural circuit that is only open at night: after receiving light at night, ipRGC will induce negative emotions through this circuit. Researchers speculate that this circuit appeared to prevent human ancestors from going out during the transition between day and night and being preyed upon by natural enemies.

To some extent, it can be said that it is the ipRGC that grows toward the light that has allowed humans to develop a life habit of working at sunrise and resting at sunset.

Just like plants, humans also need to live towards the light!

References

[1] Lok, R., et al., Light, Alertness, and Alerting Effects of White Light: A Literature Overview. Journal of Biological Rhythms, 2018. 33(6): p. 589-601.

[2] Huiberts, LM, KC Smolders, and YA de Kort, Shining light on memory: Effects of bright light on working memory performance. Behav Brain Res, 2015. 294: p. 234-45.

[3] zhu, H., et al., Moderate UV Exposure Enhances Learning and Memory by Promoting a Novel Glutamate Biosynthetic Pathway in the Brain. Cell, 2018. 173(7): p. 1716-1727 e17.

[3] Mason, IC, et al., Light exposure during sleep impairs cardiometabolic function. Proc Natl Acad Sci USA, 2022. 119(12): p. e2113290119.

[4] Burns AC, Saxena R, Vetter C, Phillips AJK, Lane JM, Cain SW. Time spent in outdoor light is associated with mood, sleep, and circadian rhythm-related outcomes: A cross-sectional and longitudinal study in over 400,000 UK Biobank participants. J Affect Disord. 2021 Dec 1;295:347-352. doi: 10.1016/j.jad.2021.08.056. Epub 2021 Aug 27. PMID: 34488088; PMCID: PMC8892387.

[5] Ma LZ, Ma YH, Ou YN, Chen SD, Yang L, Dong Q, Cheng W, Tan L, Yu JT. Time spent in outdoor light is associated with the risk of dementia: a prospective cohort study of 362094 participants. BMC Med. 2022 Apr 25;20(1):132. doi: 10.1186/s12916-022-02331-2. PMID: 35462547; PMCID: PMC9036798.

[6] Meng, JJ, et al., Light modulates glucose metabolism by a retina-hypothalamus-brown adipose tissue axis. Cell, 2023. 186(2): p. 398-412 e17.

[7] Kim, KY, et al., Synaptic Specializations of Melanopsin-Retinal Ganglion Cells in Multiple Brain Regions Revealed by Genetic Label for Light and Electron Microscopy. Cell Rep, 2019. 29(3): p. 628-644 e6.

[8] An, K., et al., A circadian rhythm-gated subcortical pathway for nighttime-light-induced depressive-like behaviors in mice. Nat Neurosci, 2020. 23(7): p. 869-880.

Planning and production

This article is a work of Science Popularization China-Starry Sky Project

Produced by: Science Popularization Department of China Association for Science and Technology

Producer｜China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

Author: Hardy, PhD in Neurobiology, Zhejiang University

Review | Wang Liming, Senior Researcher and Deputy Director of the Institute of Molecular Physiology, Shenzhen Bay Laboratory

Planning丨Lin Lin

Editor: Lin Lin