Despite common misconceptions, babies aged 12 to 18 months do not always sleep through the whole night. Instead, their sleep undergoes a series of cycles.
10-15 minutes following bedtime, your 1-year-old should settle herself to sleep. 30-45 minutes after successfully beginning sleep, the deep sleep part of the cycle has begun, typically lasting for around an hour. If your 1-year-old has not learned how to self-settle, she may wake either just before or just after this deeper part of the cycle.
Wakeful periods do occur as a normal part of your 1-year-olds sleep cycle. This typically happens at around midnight and then regular intervals thereafter into the early hours of the morning; entering an exceptionally light sleep then waking momentarily. Babies who have learned self-soothing or self-settling will find it increasingly easier to drift off to sleep by themselves after the wakeful part of the cycle. These babies have already formulated how to sleep independently.
If your 1-year-old relies on you to help settle her to sleep at bedtime with either a breastfeed, dummy, rocking, and so on, she is going to require the same treatment if she wakes during the night. The way in which you settle her for the night will predispose her expectations for wakings during the night to be able to return to sleep.
To help both you and 1-year-old have a better night, with fewer disturbances for you and your family, teach self-settling. This way your 1-year-old can become an independent sleeper. However, there is no need to leave her to cry for long periods at bedtime or during the night wakings. This simply causes distress and negative association. Instead, reassure her, then allow short periods of time for her to self-soothe. Keep returning to reassure her but do not use props such as a feed, rocking, patting and such. She will eventually learn she can sleep independently. In doing this, try to remain calm (however long it may take!).
Consistency is key! It has a vital role to play in achieving the desired result. Remember to always provide a clear message so that your baby knows what to expect.
Within 3-7 nights your 1-year-old should have learned how to get to sleep on her own and recognise her own sleep queues.