I feel the same way. I thought it was because summer time is more laid back - you know, school is out, weekends are spent at the pool and of course, more martial art training...I find that when the weather is changing there is more stress on the job, less sleep because of more responsibility is spreading me thin and so on. I could use some advice on this myself...LOL
I take different types of vitamins this time of year (some active mind and body), eat more protein, and pay close attention to my intake of omega 3 and omega 6 fatty acids for better concentration. I have heard using Sea Salt instead of regular salt helps.