Long negotiation sessions can stretch into mealtime, causing a drop-in blood sugar. Glucose feeds the brain, and we are less able to function with normal business comportment when we are hungry.
Hunger can affect the mood in many ways, such as increasing aggression. Low blood sugar creates a feeling of stress, which may manifest as aggression or disagreeable behavior. This has been confirmed in many studies[v].
In operation, the party who has more stamina may want to continue, and the other should say, “Let’s get a snack.” Preferably one that metabolizes quickly. Eat if you want to be more agreeable and close a deal; deny yourself if it is to your benefit to be ornery.