Effort cost
What an action feels like it will cost you. Dopamine helps set the price. In animals, less of it made everything cost more.
Dopamine isn't energy and it isn't pleasure. One of its jobs is pricing effort. Block it in animals and they stop working for the bigger reward and take the small easy one. They still like the reward. They just won't pay for it. Evidence: decent It also seems to set how briskly you move. Evidence: decent Parkinson's, where the cells supplying dopamine to movement circuits die, is the extreme case. Evidence: solid
Most of this comes from animals. Nobody has measured it on an ordinary evening when you can't be bothered. But it fits: when you're flat, a short walk gets priced like a long one. The walk hasn't changed. The price has. Evidence: worth trying
You'll read that exercise releases dopamine. In people, that's oversold. One brain-scan study of 12 healthy adults found no measurable change after 30 minutes on a treadmill. A small study of 16 young adults found some release during 40 minutes of easy cycling, tied to quicker reactions. Small and mixed. Evidence: worth trying You don't need the chemistry settled. In trials with people who have depression, exercise helped about as well as medication or therapy. Evidence: solid
Why it matters
The price is a reading, not a verdict. When the walk feels expensive, don't argue with it. Shrink the walk until it's cheap enough to buy, like lowering the bar for the gym. Evidence: worth trying The mood benefit is the solid part.