Overload and overreaching is common in swimming. During these periods, the nervous system undergoes high levels of stress. Luckily, there are top researchers looking into the influence of training on the nervous system.
This interview with PhD (c) in human performance and health research, Alexandra Coates (see her info and past researchhere). Alexandra is a former competitive swimmer and elite triathlete. Her and her team at the University of Guelph recently published a study: