Origin of "sleep like a baby"

I believe that many, or in fact, most of you have heard the phrase "sleep like a baby". But actually, where does the phrase originate from? Personally, I don't think using baby is a good reference as babies usually wake up crying at night or wake up and scream every 3 hours asking for milk.

I would like to ask why is the reference for babies used instead of other, possibly better references like 'sleep like a patriarch', for instance?

Solution 1:

This should be self-evident from the nature of babies.

When they sleep, the do so with a peace and softness that comes from how they are protected.

A baby may not 'sleep deep' - in the sense of 'inability to be awoken' (since they wake up crying).... but their sleep is untroubled.

Solution 2:

A baby doesn’t necessarily have to be an infant: the first definition of ‘baby’ in the online OED reads (emphasis mine):

A very young child, esp. one not yet able to walk and dependent on the care of others; an infant. Also applied to an unborn child. Formerly also: †a child of any age (obs.).

Slightly older children tend to sleep very heavily and deeply, which is why you can move them and carry them out to the car without ever waking them up—good luck doing that with an adult! There is also the variation ‘sleep like a child’.

A patriarch would be a very bad example to choose, since patriarchs are (to the extent that they still exist) the ones who have all the troubles and worries in the family, and therefore most likely sleep quite badly.

(You can also sleep like a log or a stone, which are of course things that are even more difficult to wake up.)