What does it really mean to "sleep through the night"? Chances what you think that phrase means and what your pediatrician thinks that phrase means are two different things.
While you might hope that sleeping through the night means putting your baby to bed in the evening and having him sleep all the way through until you are ready to wake up the next day, that's not the case. While different doctors define sleeping through the night in different ways, most consider it to be when a baby can sleep 6 hours in one stretch.
So guess what that means, if you are putting your baby to bed at 7 pm, and he wakes at 1 am, some doctors would say he's met the milestone. Other doctors may put time frames on the term, (sleeping from 11 pm to 5 am, for example). So before you get overly frustrated with lack of sleep, pause to consider if you baby maybe is "sleeping through the night". He may not be sleeping through the night during the same hours you like to sleep.