"Often, when forced from his hammock by exhausting and intolerably vivid dreams of the night,
[ . . . ] and when, as was sometimes the case, these spiritual throes in him heaved his being up from its base, and a chasm seemed opening in him, from which forked flames and lightnings shot up, and accursed fiends beckoned him to leap down among them; when this hell in himself yawned beneath him, a wild cry would be heard through the ship; and with glaring eyes Ahab would burst from his state room, as though escaping from a bed that was on fire."
Musing: This line 'when this hell in himself yawned beneath him,' always takes me back to Milton and Paradise Lost: “The mind is its own place, and in itself can make a heaven of hell, a hell of heaven.” Melville was moved by Milton's work. I slugged through it, but bits and pieces do remain.
Imagine how the sailors on board the Pequod felt seeing their captain flee, screaming from his bed. This description of Ahab is both apt, and the start of the fire metaphor that will stay with him throughout the book. Remember the 'fiery hunt?" Ahab himself is fire.