September Seasonal Change

What is it about September that lowers my motivation?  Is it the temperature change?  Change in sleep pattern? The moving from summer schedule to fall school schedule?  Is it the pending October 15 tax extension deadline?
Since I have done a written journal since October 2006, I have documentation of my emotional valley for both 2007 and 2008.  The excuse for 2008 was Hurricane Ike (Sept 13, 2008 to Sept 20, 2008) with no power at the house for a week -  that's solved now with our generator.
I have never been a study of biorhythms but decided to look at my graph today:

Biorhythm56days

Check out the green line (Emotional) and see the valley - 09/16/09.  Coincidence or reality?  Now look at the composite of unique pairs of Intellectual, Physical and Emotional:

Biorhythmsecondary

Mastery - Intellectual and Physical - Athletic ability and learning physical skills 
Passion - Physical and Emotional - Motivation to act, drive and sexual peak
Wisdom - Emotional and Intellectual - Presence of mind, understanding the important things in life
This chart indicates I am just coming off a medium low of Passion (motivation). 
So what does this all mean? and knowing the information what actions would you take?
Like watching the weather - all I can do is let it pass.  But if you believe the chart I should challenge someone to tennis  on Tuesday October 13!
Thanks to www.facade.com  (how appropriate) for the charts.

PS -  For the mathematically inclined:
Physical = sin(2*pie*t/23 days)
Emotional = sin(2*pie*t/28 days)
Intellectual = sin(2*pie*t/33 days)
Where t = number of days since birth