Tom Sawyer and Huck Finn would laugh their heads off if they read this one: "Sporadic Play Activity as Beneficial to Child Health as Continuous Bouts of Exercise, Study Suggests." Resaerchers at the University of Exeter measured the frequency, intensity and duration of bouts of physical activity in a group of children and analysed the results against a number of health indicators. Their results: "...a child who accumulated short bursts of moderate or vigorous exercise throughout the day was just as healthy as a child who did a similar amount of activity over longer sessions."
This may come as a surprise to those who advocate for long, concentrated, adultified exercise sessions. But children's bodies know what's best: recess breaks and frequent play sessions throughout the day. As the study author put it "If future research backs up our findings, we would do better to encourage young children to do what they do naturally, rather than trying to enforce long exercise sessions on them.