I think a lot of is from the sleepy hormones putting you straight back to sleep afterwards. Also, breastfeeding mums who are sleeping close to their babies will sync their sleep cycles so you will mostly only wake from a light sleep each time which is less tiring and easier to go back to sleep after. And of course co-sleeping mums can often feed without really waking up at all. And also that not all ff babies sleep longer I guess. Interesting, isn't it!