What Was Churchill's 'Black Dog'?

Winston Churchill, 1874-1965 was Prime Minister of Britain in 1940-45 and 1951-1955.
He referred frequently to his family and friends about his 'Black Dog' which was a euphemism for depression.
He suffered with depression for most of his life and at times struggled to hide the extent of his black moods from his political colleagues.  Interestingly he used to lay bricks as a means of distracting himself when depressed and felt that this kept the 'Black Dog' at bay.
Despite his depression he is still regarded as an inspirational leader, who refused to allow Britain to even consider surrendering to Germany during the Second World War.  
So popular was he that after his death in 1965, some 20 years after the end of the war, he was given a full state funeral.  He is also the only British leader to  have a US warship named after him.

His depression

Depression