word usage - difference in meaning "sleep for eighteen hours" and "sleep eighteen hours"

If I want to answer to how many hours the babies sleep, which of the following sentence is suitable.

Babies sleep eighteen hours.

Babies sleep for eighteen hours.

What is the difference in meaning between "for eighteen hours" and "eighteen hours"?

I would like to learn about the difference between the two sentences in general, in order to decide when to use which.