More Is Less

A riddle attributed to British prime minister George Canning, among many others:

A word there is of plural number,
Foe to ease and tranquil slumber.
Any other word you take
And add an S will plural make;
But if you add an S to this,
So strange the metamorphosis,
Plural is plural now no more
And sweet what bitter was before.

----please, draw me a sheep----