Learn Polish: Pierwsza dawka leku sprawiła, że bezsenność zniknęła. - The first dose of the medication caused the insomnia to disappear.

Unlike English, which uses "make/cause + object + infinitive", Polish causatives with sprawiać / sprawić require a subordinate clause introduced by że. You must conjugate the second verb, literally saying "caused that the insomnia disappeared."

Both verbs in this sentence are perfective (sprawiła and zniknęła). Could we use imperfective verbs in this structure?

Yes. If you wanted to describe an ongoing or repeated process, you would use imperfective verbs: sprawiała, że bezsenność znikała (was causing the insomnia to disappear). Here, the perfective is used because it was a single, completed event resulting in a finished action.

The noun bezsenność ends in a consonant. Why does it take the feminine verb ending in zniknęła?

Abstract nouns ending in -ość (like bezsenność, miłość, wolność) are feminine in Polish. Therefore, they require the feminine past tense verb form, even though they lack the typical feminine -a ending.

I've sometimes seen the past tense of zniknąć written as znikła. Is zniknęła also correct?

Yes, verbs ending in -nąć often drop the -nę- in the past tense, leading to two accepted forms. Zniknęła and znikła mean exactly the same thing, though zniknęła is generally more common in modern Polish.