I dont believe for one second that needing 8 hours of sleep is a myth. Women used to wake in the night to protect their young, but humans adapt to what circimstances they are under, and we no longer need to do that. It also used to be very common for people to sleep during the hot part of the day, which we no longer do. Mothers often used to sleep between lunch and when it was time to prepare dinner, and since we no longer do that, we DO need sleep in the night. Not only is sleep deprivation a torture method, it is a widely accepted theory that sleep greatly improves quality and length of life:
I believe each person needs a different amount of sleep, and it is up to you to figure out how much you need. I can deal with 8-10 broken hours, or 6-8 straight hours, but I barely function with 5-6 broken hours of sleep. If I get any less than that, I feel unsafe driving my kid around, unsafe using power tools or sharp knives, and often like i am going to pass out. I make stupid mistakes like burning myself while cooking, letting the sink overflow with water because I got distracted, leaving butter in a hot car, or forgetting to make an important phone call. To put it simply, if you cannot be 100% fully functional because your nights are lacking hours of sleep, then you need more sleep. Some people might exist perfectly fine on 5-6 hours of broken sleep, but everyone I know of who claims that has issues (either mentally or physically).
If you need some sleep mama, dont hesitate to nightwean a 15 month old. And regardless of what you decide, it is helpful for a baby's developing sleep habits to be latched all night long. I think at the age he is now, it is going to be very hard to teach him not to be latched on all night without some weaning happening. DD did this, and it was the reason we started nightweaning. I started just holding her when she cried, patting her back, and doing other things to comfort her and I very quickly noticed that she would fall back asleep on her own without the boob in her mouth.
I fully intend to have this new baby nightweaned by his first birthday at the latest. I need sleep at nighttime if I plan to be a good mommy in the daytime.