Why do nine-month-olds seem so tricky to get to sleep at night?
By nine months they've usually started crawling and that developmental leap can have a huge impact on their sleep. Becoming mobile is exciting for them and they want to practice whenever they can, so they tend to crawl around their cots a lot instead of lying still and going to sleep.
Babies classically start cutting teeth at around six months old until they're around two years and while the night wakings can begin because they're in pain, after a while they can be waking simply out of habit, knowing Mum will come when they cry. The difficulty for parents is knowing when they're in pain and when they're not.
By eight months you switch to solids before milk feeds and parents sometimes don't get the balance right. If the space between their solids and milk feeds is too far apart then the children can reduce their milk feeds too quickly, and the solids aren't as calorie-rich as the milk feeds. For example, a bowl of broccoli and peas is nowhere near as calorie-rich as a full milk feed. Or it may be that there's not enough protein/meat in their diet. Both scenarios can result in the child waking up hungry.
In all three instances parents fall into that accidental trap of thinking they need to start rocking their child to sleep again or patting or feeding their child back to sleep, but by nine months those techniques no longer work. A nine-month-old has outgrown being rocked to sleep and will find it over-stimulating and frustrating – keeping them awake. It goes from being helpful to a hindrance.