Dosage is a medical word, and the poem uses it to treat fear almost like a drug: something you take in small, careful amounts. The speaker lies in bed, "swelling," a word that could mean the body, the mind, or the fear itself growing as the night goes on. It's deliberately unclear, and that vagueness adds to the unease.
What stands out is how calm the speaker sounds. They "measure a little fear" and "observe exactly" what the mind does at night, as if they were a nurse or a scientist taking notes. Yet the very need to be so precise suggests the fear isn't entirely under control. Watching it closely may be a way of coping with it.
The phrase "the custom of the mind at night" makes anxious night thoughts feel like a habit or ritual, something that happens every time. The mood is quiet, restless, and slightly clinical, with fear kept small but never quite gone.