"Sleep patterns change as people age. As shown in the two graphs above, infants spend more time sleeping and spend a greater percentage of sleep in REM sleep compared with the times of older children and adults. For example, newborn babies sleep about 16 hours per day and spend about 50% of that time in REM sleep. Older people (50-85 years old) sleep only 5.75-6 hours per day and spend 13.8-15% of that time in REM sleep."
Maybe they mean just that. Maybe they mean they slept with their butt up in the air. Many babies actually do sleep through the night though. Not immediately of course... but if the parent knows what he/she is doing... the baby can sleep through the night.