Sadness
Sadness is one of the most basic and elemental human emotions, alongside fear, anger, disgust, happiness and surprise (or shock in extreme cases). Sadness is an emotional state motivated by a negative turn concerning an individual's state of mind (or mood). It's a manifestation of affective hurt expressed through crying, a weary and exhausted facial appearance, a lack of appetite, etc.
Often, we'll feel sad when our expectations aren't reached; when life circumstances are more painful than they are joyful. Happiness could be considered the adverse emotion to sadness. Sadness and melancholy can be deemed synonyms, as well as nostalgia.
Melancholy or sadness is unmistakably felt, it seizes our attitude almost completely, which validates our need to tell people and say the words "I am sad", "I feel nostalgic", etc. Be careful, however, because feeling sad is not the same depression, something to be noted as they are easily interchanged and confused in our day-to-day use of language. Both sadness and depression produce mechanisms that entail a significant wearing of energy and, akin to this wear is, the feeling one needs to rest both emotionally and physically to recover for the loss of energy, especially in the brain, where the substances responsible for maintaining levels of energy are produced. Hence, times of melancholy and sadness are usually accompanied by a drop in physical activity.
We must understand the source of our sadness and then learn to take control and learn to manage, as best as possible, this emotion before finally feeling positive and enjoying life.