Mac very slow when waking up

Hi,

Often, when I come back to my IMac either same day, or next morning, it is extremely slow. I am running activity monitor and the one thing I notice during these slow periods is that the compression memory is way up 4.5GBs or so. I was able to get the below image after the memory when down, but you can see it's pretty high before that.

It seems, that when I've left the computer (say the night before) I have to close Mail, and Chrome, etc. when I come back to clear a memory issue. After closing, I can reopen and things are pretty speedy.

Any thoughts?

Thanks - Glenn