Parents shouldn't worry if their infant doesn't sleep through the night by a year old. The authors of a study found that a large percentage of healthy babies don't start sleeping through the night even at a year old. The research team also examined whether infants who didn't sleep for six or eight consecutive hours were more likely to have problems with psychomotor and mental development, and found no association. The researchers also found no correlation between infants waking up at night and their mothers' postnatal mood. Source 1r.