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Studies have shown that colder months or extended night hours can create an impact on mental health, also known as the seasonal affective disorder (SAD). The American Academy of Family Physicians indicates that five percent of the US population experiences the disorder every year, with symptoms lasting almost five months within the year.

Norman Rosenthal coined the phrase SAD in a journal articles in 1984, and he advocates light therapy as a treatment for the disorder. The therapy involves facing a “lightbox” panel for about 30 to 60 minutes per day within autumn and winter season – a simple treatment that can be used at home.

SAD is said to be connected to the circadian rhythm of our body – waking and sleeping routine controlled by light and darkness. When the brain receives insufficient sunlight, it isn’t sure when to get up so it will continue to produce low levels of sleep hormones called melatonin. Darker days also prompt the brain to produce less happy hormones – serotonin – which is why most people feel blue when the weather is cold.

Professor Marshall Burke, along with his team at the Department of Earth System Science at Stanford University, conducted studies suggesting mental health declines when temperatures increase. The research team used a decade worth of data from Mexico and the United States and found out that suicide rates jumped by 2.1 percent in Mexican municipalities and 0.7 percent in US counties for a one degree Celsius increase in monthly average temperature.

Apart from missing the sunlight on darker days, we also miss the vitamin D that comes with it, according to the South China Morning Post. Vitamin D is essential in making our bones stronger, decreasing the risk of cardiovascular disease, cancer, and type 2 diabetes. Doctors check the vitamin D levels of a person not just for bone health, but also for the overall health and immunity of the body.

An associate professor in Hong Kong Polytechnic University’s School of Accounting and Finance, Yangyang Chen, also believes that sunlight is essential in our mental health. He demonstrated in his recent study that patent inventors are more productive when exposed to sunshine, while stock market investors were pessimistic on cloudy days.

Chen explains sunlight creates a huge impact in a person’s mood. For instance, serotonin levels are higher on sunny days than cloudy days, so low levels of the hormone mean the emotional state is elevated. Melatonin also increases during extended night hours, and the hormone is linked to fatigue, depression, and sleepiness – altering the mood state just like drugs with sedative-like properties.

SAD is usually common in high-latitude countries like Canada, Russia, as well as Northern Europe. And even though Hong Kong is at a low-latitude, Chen said SAD might likely be prevalent in the city as it experiences a lot of cloudy and rainy days. He said an insufficient amount of sunlight leads to bad moods, making people unhappy on colder days, so they tend to be pessimistic towards life and work.