Tell your companions what you need before fatigue turns it into resentment.

Needs stated early sound like information. The same need stated three hours later, through tiredness, sounds like an accusation, and by then it has usually become one. The distance between noticing you are hungry and being unable to be pleasant is much shorter than most people plan for.

Say the small thing at the moment you first notice it. You need to sit down, you would rather eat somewhere quiet, you have reached your limit for shops. Early is easy to accommodate, and late rarely is.