This is your body telling you what your body needs. Listen to it. This is not a race, this is a baby your body is making..
For your health and the health of THIS baby, you need to rest.
This falls in the normal range. You are older, your body has already been through other pregnancies and each one can be very different. Did you know, it can take a woman's body up to 7 years to totally recover from a pregnancy?
Add to that all the things you do to push your body.. and at some point, your body is going to say hey. I need a rest.
Your husband and children should be honored to allow you to rest and take care of this new member.. Just be honest. "Mom is tired so we are going to have a quiet time while I nap."
They will be sharing you soon anyway, they might as well get started now.