First of all, I did some research, and didn't find anything related to my question (a special issue I'm having I guess).
Second, I know that Android uses as much Ram as possible, and the more ram it uses, the better it is (but that's not my case).
Here's the problem :
It happened last month, and will happen soon again.
With time, Ram consumption increases in my Nexus 5 daily, bit by bit until only <100Mo is free. This leads to the keyboard not opening up for example, because the Ram is full.
Compared to my Nexus 4 (rooted, PA 4.2b4 [even though I don't think it makes any difference]), apps stay active >200 hours (facebook, viber, ebay, Swiftkey etc.), while on N4, Swifkey is active since 22hours ago for example.
I don't really reboot my Nexus 5 (stock, unrooted), maybe this is the issue ?
When it happened a month ago, I uninstalled a game and it fixed it, until now.. Available Ram is roughly 600Mo, and it's shrinking each day.
I hope I didn't forget anything, if you think I did, please do ask.
Thank you for any suggestions (and for reading this, while you're at it.)