Joe Friel recently posted an article reporting on a study comparing a standard linear periodization training model vs a block periodization training model. Go check out his great article on Block Periodization. The basic premise of the study was to determine whether athletes who followed a linear training plan where they completed workouts and rest fairly evenly interspersed compared with athletes who followed a plan which placed a high volume of workouts close together within the first week of the training block with minimal rest followed by a several weeks with only one high intensity workout and I high volume of recovery workouts.
The study found that athletes who followed this block type training schedule experienced greater fitness gains over the training period even though they completed the same total volume of high intensity and low intensity work.
This lends weight to the idea that it is very important both how you train but also WHEN you train. Joe draws some conclusions as well based on the research that was done. This may help you add a tool to your toolbox or better understand how training affects fitness. I hope it will help you.