His opening monologue, a contemplation of how elusive sleep is to mighty kings, showcases the breadth and depth of Shakespeare's imagination. Starting with the complaint that he, lying peacefully in the cozy and "perfum'd chambers of the great" (12), cannot get any rest, King Henry goes on to imagine where sleep has landed tonight: in peasants' shacks, choked with smoke from the fire and interrupted with "buzzing night-flies" (11); or even with the boy at sea who, perched in the highest mast of the ship to keep watch amid the tumbling of the waves, the blowing of the wind, and the wet spray from the ocean, still manages to doze off (18-31). Henry asks rhetorically, "Canst thou, O partial sleep, give thy repose, / To the wet sea-boy in an hour so rude, / And in the calmest and most stillest night... / Deny it to a King?" (26-30). He closes the speech with one of the play's most famous lines, often quoted as a motto about the anxiety and discomfort that accompany great power: "Then happy low, lie down! / Uneasy lies the head that wears a crown" (30-31). This kind of speech--the dark and uneasy contemplations of a king, alone in the middle of the night, facing the darkness in his own soul--seems to have fascinated Shakespeare. (Similar speeches appear in Macbeth and Hamlet.)
The king's second speech, after the other noblemen come in, is also grand in scope. Northumberland's rebellion has caused him to think about his own earlier rebellion, eight years before, in which Northumberland was his ally. His realization of how everything changes causes the king to despair on a grand scale: he wishes he could get enough perspective to "read the book of fate" (45) and see how time has made mountains wear away, coastlines disappear into the ocean, and water covering dry land.
Richard also seems to be a worried by his memory of the prediction that King Richard II had made, when Henry himself overthrew him: that Northumberland would rise against Henry himself. (The events Henry recounts in the passage are covered in Shakespeare's Richard II, III.iii, and V.i.) Warwick calms Henry down by reminding him that Richard's prediction was not a difficult one in light of his betrayal by Northumberland. Still, the events of the past clearly loom over this play, haunting both Henry IV and his antagonists. The king's repetition of his desire to go to Jerusalem, for instance, is a theme continued from both Richard II and Henry IV, Part 1, which will eventually have its resolution in this play in IV.v.