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.
Source:
Durmer, JS et al. "Pediatric Sleep Medicine". Continuum. Neurol 2007; 13(3):153-200.
