New parents are often told never to wake a sleeping baby. While this advice is no doubt well intentioned, it can, if followed, lead to substantial night time sleep disturbances which in turn can have a really negative effect on the sleep of the parents and the rest of the household.
While it does make sense not to proactively disturb a sleeping infant during the first few months of life, once a regular day/night circadian rhythm develops (usually between 3-6 months of age), there is no reason why babies and older children should not be getting most of their sleep at night, and only a small (and diminishing) portion during the day. The more one sleeps during the day, the less one needs to sleep at night, and this is true for everyone, young and old. After sleeping a certain number of hours, one can sleep no longer, wakes up, and has great difficulty falling back to sleep. This daily sleep requirement is roughly constant, though it varies between individuals, changes with age, and is influenced by factors such as sleep deprivation, alcohol, and medication. A good way of understanding this is to think about the 24 hour day as a pie from which a certain amount must apportioned for sleep. One can cut a single thick slice, or several thin ones, but the total amount removed from the pie stays about the same. And so, if one takes a two hour nap in the afternoon, it becomes more difficult to fall asleep that night. By the same token, an increase in the amount of sleep a child gets during the day reduces her need to sleep at night, and results in increased wakefulness. While this doesn't necessarily cause any hardship to the 14 month old who awakens and wants to play with her parents at 2 AM, it can be difficult for them if they need to wake up the following morning and get to work on time. Therefore, maintaining and enforcing a regular and balanced sleep schedule winds up making everybody's lives easier, even if it means keeping that toddler from taking a three hour nap in the middle of the day.