Most infants achieve this by 5 months. By ‘sleeping through’ this means they obtain 6 hours (or more) of sleep. Typically babies need to eat at other times during the night. There is no research to support that a baby of x weight or by 6 months should be sleeping 10 – 12 hours. The developmental goal at 5 months is 6 hours of sleep.
Sleep can be difficult for colicky infants, fussy babies or PURPLE crying is all normal behaviour and take heart that all humans go through to varying degrees. It usually increases in the afternoon and evening. Strategies like deep pressure massage and a deeper bath (with extra vigilance & never leave a baby unattended) can help calm your baby before bedtime. Identifying calming strategies that work with an individual baby will help you get your infant to sleep.
Feed your infant at night in a dimly lit room, change his or her diaper and put them back to sleep. Help your infant sleep better by getting outside for a walk during the day and conduct daily activities in a brightly lit room. Most babies learn to decipher day from night in the first month. After this time, they can be put for their naps in a dark room during the daytime for better sleep.
You likely want to get your infant to sleep through the night. Help facilitate this by bringing your baby to the breast more often (e.g. every 2 or more hours) in the early evening helps your baby store up on much needed calories so if they are ready to sleep a longer stretch they will be able to achieve this. The hind milk, which makes up more of the proportion of breast milk in the afternoon/evening, is made from rich fatty milk that has lots of calories to help sustain longer sleep for infants. Introducing formula and/or solids has not been shown in research to help with sleep.