The past perfect tense of the verb ‘sleep’ is ‘had slept’.
This tense is used to describe an action that was completed before another action took place in the past. For example, you might say, ‘By the time I arrived at the party, everyone had slept for hours.’ This indicates that the sleeping occurred before the arrival at the party.
To form the past perfect tense, you use the auxiliary verb ‘had’ followed by the past participle of the main verb. In this case, ‘sleep’ becomes ‘slept’, making ‘had slept’ the correct form.