Why might you feel sad when theres nothing to be sad about sometimes?

Fatigue.: Our emotions and our physical state of being are intimately connected. Feeling sad can be, and often is, the result of fatigue. A sad feeling can easily arise when there is "nothing to be sad about." when this happens take it as a sign that it is time to pay more attention to lifestyle; nutrition is a possibility, but most often the focus should be on sleep.