Now consider the mental health problems that we know are correlated with low amounts of serotonin in the neural synapse: depression, anxiety, attention deficits, perseverations and so forth. Please note that I am not saying that it is the lack of serotonin in the synapses that cause all of these--in fact, what happens is an imbalance in a number of transmitters due to developmental differences in the brain's structure that make a person susceptible to these neurological problems--but we know from research and drug therapies that increased serotonin in the synapses greatly alleviate these problems and that environmental factors such as ongoing stress increases them. Furthermore, we know that many neurological syndromes such as ADHD, OCD, and Autism Spectrum Disorders (ASD), share symptoms such as anxiety and depression and perseverations.
Our children need and deserve plenty of time playing outside in the physical environment. Among mammals, play is important work that develops strength, reflexes and ultimately, develops the brain and the intellect. This is not just leisure activity to be used to fill in the time when they are not learning. As John Holt used to say, we are "learning all the time."