Nuthatch behaviour?

Has anyone noticed that Nuthatches appear to freeze for a few minutes every so often?

I was watching one earlier and it landed on a branch above the feeding post and just stayed there, moving its head now and again and keeping an eye on me but essentially, frozen still.

I've seen this before. Is it normal behaviour? Having a wee rest, sleep?

Uncropped photo, it let me this close (18-600mm)