St. John's Wort is a flowering plant named for its bright yellow flowers that were said to bloom for the first time around St. John the Baptist's birthday. People have been using St. John's wort for centuries.
St. John's wort is most commonly used for "the blues" or depression and symptoms that sometimes go along with mood such as nervousness, tiredness, poor appetite, and trouble sleeping. There is some strong scientific evidence that it is effective for mild to moderate depression. One reason people may wish to try the natural remedy for depression (as opposed to antidepressants that can increase serotonin) is that St. John's wort tends to have fewer side effects than medications.