The word Luna refers to the moon. The Romans believed that the moon's phases can cause a person to go temporarily insane. A person who had gone insane because of the moon was supposed to be “moon-struck”.
In Latin lunaticus meant to be moon-struck. This word later got absorbed by English and became Lunatic.
While modern science has written away the connection between the moon and mental health as ancient lore, new research is beginning to show that it might not be entirely untrue. Research carried out by Dr. Wehr at the National Institute of Mental Health in Bethesda tracked patients with bipolar disorder. He found that moods undergo a cyclical change in the patients.
The reason for this is still unknown. The causes could be a shift in magnetic fields due to the moon or because of the moon's gravitation. Much more research is needed to determine this for certain.