Read these sentences from the passage.
But as the weeks went on, and the people had to work and toil continuously in the fields, their crops grew in such abundance that they could scarcely keep up. And because there was only daylight, it was always hot, so hot that sleep was impossible. The people, already tired from long hours of work, became short tempered and irritable. Everyone was on edge.
What does the phrase "on edge" mean in the last sentence?