agree

In addition, your age matters: are you exactly 40? In the low 40s? High?

I'd say if you are mid or high 40s, forget switching out of IT (unless like 老土 said nursing is your dream job). My personal observation is that IT has at least another 10 years of good time, so for your age you do not need to really worry about IT as a whole for a job market, rather take care of making yourself marketable.

If you are 40, you can consider the switch. Be sure to be aware, as others said, that nursing is a quite physical work and your age 15 years down the road may work against you. Also, I think to become a nurse you do need some formal training. I know that in college nursing is a major so it take some 4 years for becoming a nurse. YOu may not necessarily need to do that, but I highly doubt that you can just read a book, take an exam and them be a nurse.