The poem imagines a dead skull, buried in soil, slowly turning into a place where new life begins. A small animal passes through and leaves an acorn in the brain case. The acorn wakes up, and the line "can't go back" suggests that once it has started growing, there is no returning to sleep. That is where the title comes in: insomnia here is the restlessness of a mind that has been woken and can't settle, though now the "mind" is a seedling.
The speaker doesn't seem horrified. The tone is calm, observant, even a little wry. Death is treated as ordinary, something that happens "again," and the skull is simply one more patch of ground. The ending joins thinking and growing together: fungal threads replace neurons, roots search for light, and the only watching eye belongs to an owl. The mood is eerie but oddly peaceful, as if consciousness doesn't end so much as spread into the living world around it.