They examined 48 healthy men and women, age 18-25. Half of the group did a 20 minute, moderate intensity workout on a stationary bike, the other half did no exercise. Participants filled out mood questionnaires at the start and 1, 2, 4, 8, 12 and 24 hours afterwards.
Everybody has exercised at some point in their life. But people who don't exercise often tend to forget how good they have felt afterwards.