I am not a parent but I have experience with short sleep cycles when racing yachts in long ocean races. A watch is around 4 or 6 hours on deck, and then 4 or 6

I am not a parent but I have experience with short sleep cycles when racing yachts in long ocean races. A watch is around 4 or 6 hours on deck, and then 4 or 6 hours to eat and sleep a bit before going on deck again. This can last for a few days or a few weeks. I am always amazed at how quickly my body starts to treat each on/off cycle as a biological day. It takes around a day to get used to it, and a day to go back to normal when the race is over. But I feel like I could continue in that pattern forever, when doing it. Apparently long distance solo yachtsmen will sleep 10 or 15 mins out of each hour, sometimes for months, with little ill effects. Humans may need a certain amount of REM sleep per day, but my experience is that is doesn't necessarily have to contiguous.