A medical professional has issued a warning that a "common" deficiency could be the cause of your low mood. The expert suggests this issue affects over one billion people globally. As winter approaches, many people notice a shift in their mood. The chilly weather and early nights can often leave us feeling somewhat gloomy. In some instances, this could be due to what's known as seasonal affective disorder (SAD) . However, there might be another explanation. In a video shared on the social media