eCPM decreasing and increasing in a cycle.Why?

eCPM seems to go down and again go up like a repeat cycle

[sriraj]

I really have strange experiances with Adsense. Most of my Adsense earning come from an ad content placed above the article content.
Generally I use a large rectangle in that area and my earning seems to be smoothly increasing and finally reach a point from where it comes down. Afraid of going even more down, I replaced it with normal rectangle 300x250. After which the downturn stops and upward cycle begin and this keeps repeating all the time.The time frame being approx 2 weeks for each half of cycle. The impressions remains the same all time.
Is this normal and does everybody do something like this?
I mean is there any logic behind keep exchanging large rectangle with 300x250 ad?
Will there be anything like eCPM reaching the bottom and going new heights after?

[HowYesNo]

my ecpm is like this /\/\/\/\/\/\
and i have same traffic, ads and cpm for months

[sunhapol]

my ecpm has declined this month .. last month was ok ...

[VedranKovac]

sriraj - my 2c: i think swings in MY ecpm are driven by time mostly. as the day starts (pacific time) my ecpm is strong and goes down by the end of the day. the same happens with within a monthly cycle, the beginning of the month tends to have higher ecpm and it wears of as the month progresses. i am not sure if this is what's behind your swings, yet it strongly correlates with swings in my ecpm. finally, if you used adwords you may have a better idea why this may be the culprit (e.g., you set campaign budget limits for a day - i am not sure if you can do it for a month but again, most marketing departments manage budgets on a monthly basis anyway so this makes sense to me as well). cheers, v.k.