"Had been" implies that you have not finished the essay, but that you stopped. "Was" is past tense for "is." You probably finished, although it's unclear. "Wrote" definitely indicated you completed the essay. I hope this helps somewhat.

In this situation, past continuous sounds most natural and common. Past perfect continuous would be OK too (and would sound more formal); but it would not be necessary because the sequence in time - first writing, then sleep - is obvious. The preposition "before" makes this clear, and so the sentence sounds more like a narrative. In the following example, it is necessary to use past perfect continuous: "I had been writing my essay for 30 minutes when the phone rang." The timing of the two events (writing and ringing) is very precisely defined by reference to two past actions and so it is necessary to use the past perfect for the earlier one.

Thanks a lot for your answers. It helped. Cheers.