My brain senses a stressed senior girl right now...

Wow, what an amazing poem! Upon first glance of the poem, I was fairly lost as to what approach to take when tackling it. However, once I read each stanza carefully, I realized the poem was telling a story and the central theme was one's decent into mental insanity. Even though many feel the character of the poem is not aware he/she is mentally unstable, I beg to differ! In the first stanza, the character is trapped inside a wooden box (or a casket). Since the character is trapped, but still alive, he/she is rather a prisoner in their on body. Throughout each stanza, there is a reference to the character being helpless to the happenings outside the wooden box in which they are trapped. At the end, when the poem reads, "and then a plank of reason broke", this is referencing how a plank in the bottom of the casket broke and then the character was able to escape the imprisonment of their own body!