Newborns actually sleep quite a bit, but the problem (at least for parents) is that they have not developed a circadian rhythm, and their sleep is very fragmented. As a result, their sleep periods are brief and these are scattered throughout the day and night.
The sleep periods of newborns typically last from 2 to 5 hours. Between these sleep periods may be a period of wakefulness that lasts from 1 to 3 hours. During a 24-hour period, a newborn may sleep from 10 to 18 hours, with the average being close to 15 hours. Their sleep is often disrupted by a desire for feeding.
Fortunately, by the age of 8 to 12 weeks, these rather short sleep periods become more consolidated and more of a newborn's sleep begins to occur at nighttime. The sleep of children continues to evolve throughout their development until it approaches that which exists in adults, usually by late adolescence.
Durmer, JS et al. "Pediatric Sleep Medicine". Continuum. Neurol 2007; 13(3):153-200.