Noncount nouns never seldom take an indefinite article (a or an) before them (refer this)
- Have a sound sleep.
'Sleep' is enclosed with 'sound'.
- He has a great sense of humour. (Not "He as a humour")
'Humour' is enclosed with 'sense'; sense is countable ('great' is an adjective to describe 'sense'). "A sense of humour" is also acceptable.
More examples:
- A bowl of rice
- A lot of homework
- A piece of clothing
- A clump of cotton
- An ounce of toothpaste
Indefinite articles can be used when enclosing noncount nouns with other articles such as an adjective or descriptors.
Edit:
There are some idiomatic phrases that include noncount nouns with infinitive articles and also countable.
Three more sleeps till Christmas
This refers to the three nights that pass by.
Have a sleep
This is also acceptable in practice
Credits to Andrew Leach and Bill J for the counterexamples.