What does it mean when the blood vessels under the tongue appear dark?

Dark discoloration of the blood vessels under the tongue may be due to superficial sublingual veins. After tissue metabolism, venous blood has lower oxygen content, causing it to appear darker in color. It could also result from irregular lifestyle habits, such as frequent late-night sleep or consuming large amounts of spicy and stimulating foods like hot pot or grilled dishes, leading to "heatiness" in the body. Additionally, sublingual blood stasis caused by trauma, qi deficiency, or other factors may also lead to this condition. It is recommended to seek timely medical evaluation for accurate diagnosis.