Not sure if this has already been posted by someone, but I just came across this article about sleep on the BBC News website: www.bbc.co.uk/news/magazine-16964783
Supported by research, it argues we used to sleep in two stretches with a break in between rather than 8 hours straight which is the current norm/recommendation.
The most interesting bit to me is about 3/4 through the article where it talks about children and how it is natural for them to wake up after their 'first sleep' and not feel the need for more sleep. The source (1829 medical journal) then argues this is undesirable and they should be encouraged to go back to sleep.
What I'm gathering from this article is that it's actually NORMAL to wake up in the night, but that our social norms require us to sleep for a long stretch so that's what we learn to do from a young age. So much so that by the time we are adults and our babies are not sleeping through the night we get extremely frustrated, but our babies are just following their biological clock, it's us who've learned to override our own biological clocks.