She perceiveth that her merchandise is good: her candle goeth not out by night.
The Proverbs 31 woman knows that her merchandise is good; other bible versions I've seen say "profitable" instead of "good." Whether she's making goods, growing food, handling financial matters, or just plain taking care of things around the household, she knows that she's giving her best. And "her candle goeth not out by night." Even when the sun goes down, her job is not done. I think of us mothers today, we don't always get to have a peaceful night's sleep do we? It was the same back in the days of the Proverbs 31 woman. She didn't shirk her duties just because it was night time. If something needed to be done to care for her household, she did it, no matter what the time.
Putting the kids to bed doesn't automatically mean we get to "clock out for the night." Even if our kids are old enough to sleep through the night, sometimes they get sick in the middle of the night, or they have nightmares. I used to LOVE my sleep, and before I had kids, I used to wonder how I would fare with my sleep being interrupted, and if I would be able to be loving enough to be comforting when that happened. :) I still love my sleep, but I don't wonder about being able to comfort my kids' middle-of-the-night woes...I just do it. God is faithful to equip. :)
For some of us, kids aren't the only things we need to tend to late in the night. Sometimes, that is the only time I have to get done any housework that I didn't get done with the kids earlier in the day. When you have your kids working along-side you as you clean, not as many things seem to get done. ;) Some women also may have an at-home job to help out with the bills that they can only work on at night after the kids are in bed. Or they have hobbies that help bring in a little income during tough times.
There could be many reasons why wives and moms need to burn the midnight oil. The point of this verse? Know in your heart that your doing your best, and "do what you gotta do" to take care of your family and household.