An article investigates the precise effect of fatigue and physical state on the perception of environment we have, this is published in Perspectives on Psychological Science. We measure a distance farther and slopes inclined more when we are tired and stressed out. This is attributed to the fact that our instinct directs us to save energy and so we tend to walk slower or have a pace less than normal.
Participants were asked to estimate the steepness of hills, both from the bottom and at a cross-section, before and after a physically demanding run. The results displayed a trend for participants to estimate the steepness of the hill as greater after completing the run.
Similar over-estimation occurred when participants were asked to estimate the steepness of hills when wearing a heavy backpack, or when the participant was elderly or otherwise less physically able. Another experiment placed participants at the top of a hill steep hill, standing on a skateboard. The perceived risk of bodily injury led the participants standing on skateboards to estimate the slope of the hill to be greater than their counterparts on secure ground.
The author concludes that "what one sees in the world is influenced not only by optical and ocular-motor information, but also by one's purposes, physiological state, and emotions."