Sleeping or Asleep?

It's more usual to say that somebody is asleep than that they are sleeping. So, rather than say somebody is sleeping, you now know better! 

He is sleeping. 

He is asleep. 

However, if you want to use an adverb to describe or indicate the manner in which somebody is sleeping, you use sleeping. 

He is sleeping deeply 

He is asleep deeply. 

In the same vein, it's important to state here that certain verbs such as understand, hear, see, etc. are referred to as STATIVE VERBS and therefore are not expressed in the continuous tense (that is, with the -ing) when used in a sentence. For example;

I am understanding the concept.

I understand the concept. 

Can you hear me?

Are you hearing me?