I was thinking what could have caused his irresistible drowsiness, when I felt my brain becoming stupefied.

In spite of my efforts to keep my eyes open, they would close.

A painful suspicion seized me.

Evidently soporific substances had been mixed with the food we had just taken.
Imprisonment was not enough to conceal Captain Nemo's projects from us, sleep was more necessary.

I then heard the panels shut.