"A few years back, Frankland and his wife Sheena Josselyn—also a neuroscientist at the Hospital for Sick Children—started to notice that the mice they studied performed worse on certain kinds of memory tests after living in a cage with a running wheel. As the couple knew, exercise on a running wheel promotes neurogenesis—the growth of whole new neurons—in the seahorse-shaped hippocampus, a brain region that is essential for memory. But while neurogenesis in the adult hippocampus likely contributes to the ability to learn and remember, Karl Deisseroth of Stanford University and others had suggested5 that it might also necessitate a certain amount of forgetting. Just as a forest has room for only so many trees, the hippocampus can hold only so many neurons. New brain cells might crowd the territory of other neurons or even replace them altogether, which could in turn break or reconfigure the small circuits that likely store individual memories. Perhaps, then, the especially high rate of neurogenesis in infancy was partly responsible for childhood amnesia."
And this also explains why I always try to schedule my long runs after the calls to my mother (they are stressful), and add more miles if I have more stress in my life in general:
"when adults ran on a hamster wheel after the shocks—thereby stimulating neurogenesis—they started to mirror infants in their forgetfulness. Prozac, which also encourages neural growth, had the same effect. Conversely, when the researchers hindered neurogenesis in infant mice with drugs or genetic engineering, the young animals formed much more stable memories."