Just build a bedroom with plenty of beds but very close to eachother in a settlement of mine. I never see anyone sleep in them and now all of the sudden my happiness is getting lower. All my fields are green (water, food, defence etc..) as well as bed.

Might the beds being "too close to eachother" affect happiness?

NPC's hate tight spaces. Just give the beds a little bit of space and you should be good to go!

You just have to give them enough room to walk between beds. All of my settlers are able to sleep here- http://i.imgur.com/jxfE0tE.png

Hmmmm, my bunk house has all the beds very close to each other and they work just fine. Only weird part is seeing so many people all sleeping on their same side.

I turn my beds opposite each other just so they sleep on different ends. I was also freaked out by this, so I fixed it.

If you place beds side by side, you need to check that there is enough space between them so that you can move freely; if you line them end to end it's not as much of a problem.