As a SAHM I have a different take here. You may think you are getting enough sleep at night, but having actually had children myself I know this: when you watch children all day, you are constantly being attentive to the needs of others. You are watching for problems, choking, need for diapers, need to intervene, making sure they are all present and accounted for, handling all the household tasks, the ringing phone, the blaring tv, the dog barking, the yelling, the incessant need for attention and cuddling from a small child, the laundry, the car needing gas, bringing the food in from shopping, preparing breakfast, lunch, dinner, cleaning up breakfast, lunch, dinner, more diapers, the broken washing machine…
I could go on. In short, you may have gotten enough “sleep” the night before (or not) but during the day you can go on sensory overload without the chance to pause, collect your thoughts, or sit back and relax for three minutes before the next responsibility takes center stage. One of my sisters couldn’t go to the bathroom alone even for two years because her little son had the world’s biggest mommy attachment and would bang on the door and scream until she opened it. (Happily, he is quite over that stage now.)
Your need for a nap in the middle of the 24/7 job you do (and no man who has worked in an office and comes home expecting dinner on the table can quite understand this) is NOT an indulgence or laziness or a sin, or a misuse of time that could be spent more productively. When you’re always on duty as a mother, you ARE spending your time productively. If the kids are still alive at the end of the day and the house didn’t burn down, you’ve done your job well!

Now do it again tomorrow and the next day and the next for the next 18 years.
I understand completely that it actually feels GOOD to go lie down and rest the unseen strains in the back and neck muscles from hoisting children and laundry and groceries all day, and to take a few minutes and let the mind go blank (if you don’t lie there and plan your shopping list for later or balance your checkbook mentally). That nap that some may think is overindulgent is a necessary refresher before you start up the rest of the day and all of its repetitive but necessary activities.
Don’t feel guilty. Even Christ knew He needed to take time apart from His followers for rest. It will make you a better, more patient mother.