In fact, some of the Chinese fans of sunbathing wear head-to-toe sun-protective clothing when enjoying the sunshine as their aim is not to tan their skin, but to boost their yang qi - a vital energy in traditional Chinese medicine (TCM).

Sunbathing has become a much-discussed topic among Chinese youngsters on domestic social media platforms, despite the record heatwaves across the country.

The Global Times found more than 40,000 online search results for bloggers who promote this therapy or TCM doctors explaining the theory behind it and teaching people how to conduct the therapy at home.

"It is a very simple way for people to keep in good health at home," a TCM doctor named Mo Xiaofeng was quoted as saying in a short video on lifestyle platform Xiao Hong Shu.

"I have got sun on my back in the morning for a week. I sweat a lot and feel so comfortable when doing it. I usually feel very cold in winter and always feel a lack of energy, but after sunbathing, I feel warm to the bones and have more energy," a Xiao Hong Shu user whose account name is Han said in a post on July 15.

In Zhongshan park in Shanghai, every afternoon the grass is full of youngsters lying on their stomach to enjoy the sunshine. Two local female residents surnamed Huang and Zhang said they began doing it three years ago. It is a good way to remove cold and get yang qi into the body, they said.

In TCM theory, san fu is considered a particularly suitable time for treating illnesses. San fu refers to three 10-day periods that are predicted to be the hottest days of the year. The three periods are separately called tou fu, zhong fu and mo fu.

The basic principle for this is the yin and yang theory of TCM, which believes the balance of both elements in the body is vital for good health. According to the TCM theory, when the weather is extremely hot, the human body has higher immunity due to the vibrant yang - a driving force of biological activities in the human body in TCM theory - and blood circulation.

Usually in the san fu periods, some Chinese people will use a san fu tie - a bandage made of traditional Chinese herbal medicine, which originated in the Qing Dynasty (1644-1911). It is placed on various acupuncture points in the body to treat illnesses that are caused by too much yin qi or coldness in the body.

San fu tie contains a paste of herbs that are "hot" in nature, and when applied to specific acupuncture points, usually on the back and neck, they replenish the yang elements.

But these traditional therapies have also encountered some controversies. There have been reports of children being burned by the san fu tie and people getting sunburned after sunbathing.

Zhou Jipu, a doctor from Beijing Hospital of Traditional Chinese Medicine, warned that people with weak bodies or sensitive skin should not sunbathe.

Doctors suggest sunbathing before 9 am or after 5 pm for 15-30 minutes, and avoiding strong sunlight.