Newborns sleep about 16 out of every 24 hours.
Newborns feed about 8-12 times every 24 hours and not in perfect intervals. So while they sleep a lot, they wake frequently. There is sometimes a 4 or 5 hour sleep stretch, but newborn babies can also sleep for just 45 minutes or an hour at a time.
A newborn will move through a sleep cycle (light and deep sleep) within 45 minutes to an hour. Adult sleep cycles are twice as long. Babies do sleep for longer than 45 minutes at a time of course. This just means that they have risen up into light sleep at around 45 minutes and then drifted back into another sleep cycle. When a baby is sleeping deeply she is less likely to wake up, even when you move her. But if she's in very light sleep, she could wake easily.
Sleeping "though the night," which is considered only 5 consecutive hours for a baby, means the baby has gone through a few sleep cycles without waking all the way up. Of course all this means that newborns can't be awake for very long, either- really just a half hour of face time with an adult and a baby can feel wiped out and need to sleep. Babies tend to go through a sleep cycle, a feed and about 45 minutes to an hour of awake time before it's time for another sleep (eat and awake time), though not always in that order.
For more things you should know about newborn sleep, visit Being Pregnant.