"had slept" is the correct past perfect form of the verb "sleep." The past perfect tense is formed by the word had plus the past participle. Sometimes, this past participle corresponds with the simple past tense form:
I walked. I had walked.
I slept. I had slept.
In other situations, the past participle differs from the simple past tense form:
I knew. I had known.
I went. I had gone.
Logically, we use this tense to indicate an event that happened BEFORE another event in the past. We could use "I had slept" in the following sentence:
I had slept for only two hours before my alarm clock woke me up.
Hope that helps!
