I don't agree with this. I took a years maternity leave after a difficult pregnancy. I was absolutely trauma ridden, and I was permanently burnt out.
I did need just as much sleep as my working partner because looking after a child needs cognitive acuity, and to have cognitive acuity, you need healthy neurotransmitters which you can not produce when you're heavily sleep deprived or viewed as less than important.
I'm also not someone who can nap at the drop of a hat. Not every body can. I go beyond tired to the point of insomnia and no doctor would help me with medication in case I slept through my baby needing me.
I couldn't "sleep when baby sleeps". He barely slept any way!
If I'd have continued to work as well, I wouldn't have had more support from my partner.
Yes, someone will always be doing more childcare than someone else, but having your basic needs met shouldn't be divvied up by who leaves the house to do paid work and who stays home to do unpaid work.
And if your partner will not support you to the best of their abilities to have your needs met, or you're that partner, well, it's not a partnership.
That's what's happening here with OPs son and gf, but OP thinks the gf is a moody little madam with a scowl on her face for no good reason but can't seem to put 2 and 2 together than she's being wholly judgemental.