What is the effect of the words poison and pollutes in this stanza?

They emphasize the idea that thoughts can have negative effects.

They illustrate the negative effects of not getting enough sleep.

They show that negative thoughts alone are able to destroy one's life.

They suggest that human beings tend to be inherently negative.

Read the second stanza from "Mutability."

We rest.-A dream has power to poison sleep;

We rise.-One wandering thought pollutes the day;

We feel, conceive or reason, laugh or weep;

Embrace fond woe, or cast our cares away

The words ‘poison’ and ‘pollutes’ in the second stanza of the poem emphasizes the idea that thoughts can have negative effects.

Percy Bysshe Shelley in her poem "Mutability" touches upon the theme of thoughts that linger upon human’s mind. These thoughts affect more when a person is asleep. It is during the sleep that the thoughts start to control the subconscious mind. The theme of lack of freedom finds its place in the poem too. When a human is in sleep he loses freedom of thinking and analyzing things according to him. Thus, the external thoughts which are negative in nature start to conquer the mind. They ‘poison’ and ‘pollutes’ a mind in its subconscious state.