Questions & Answers about Jestem zmęczony, więc muszę poleżeć i pospać.
How does the prefix po- change the meaning of leżeć and spać in this sentence?
The prefix po- adds a delimitative meaning, which means performing an action for a limited, usually short, amount of time. While leżeć just means to lie and spać means to sleep, poleżeć and pospać mean to lie down for a bit and to get a little sleep.
Are poleżeć and pospać considered perfective or imperfective verbs?
They are perfective. Even though they describe an action that takes a duration of time, the prefix po- packages that action into a single, completed block of time. Because they are perfective, you cannot use them in the present tense to say you are currently doing these things.
Could I just use the imperfective verbs leżeć and spać in this sentence instead?
Grammatically yes, you could say muszę leżeć i spać, but it completely changes the tone. It sounds much heavier, like a strict doctor's order for bed rest. Using the po- forms makes the statement casual and temporary.