I notice horizontal waves on my screen from bottom to top time to time under certain conditions, mostly visible when the screen has uniform color.You can try it with paint and use 127/127/127 (dark grey) and see if you get it. I can see it when I am typing this message with the white text box area. I am not sure if it is refresh rate or just normal operation. It doesn't bother me too much (yet) but I wanted to know if there were any adjustments I can try.

Probably not. IPS screens have lower refresh rates than TN screens. Most people don't notice any flicker, but some do.

Is it 50hz?
I just played around with settings, and intel display properties have 50hz and 60hz options. I didn't want to try 60hz, but it is available there. Has anyone tried it? I don't want to fry my display board.

60Hz should be fine.

You may want to try this test:
http://www.lagom.nl/lcd-test/inversion.php

Does it change with different brightness setting ?

It is very likely caused by this LCD screens don't flicker - or do they?

Yes, it does, but it is hard to pinpoint if it actually gets more noticeable at certain brightness or if there is something else that is causing it to get wavy like lower battery level? Higher processor? Fan spinning? Who knows. Under most conditions I barely notice it, it is only when I have a light gray or mostly unisome light color image.
While seeing the waves in regular screen, I did not see any problem using the link Colonel sent.

I will try 60hz and report back later.