C'est l'heure de or du

This is just a minor point which I'd appreciate a clarificiation:
Just going through unit 2.6 which gives two examples of "c'est l'heure...":
C'est l'heure du déjeuner !        =     It's lunch time!
C'est l'heure de dormir.             =      It's time to sleep.

In the first case we have "du", which has an implied definite article, but not in the second case.
Is there a subtle difference in context, or are both correct, or is one incorrect?
Pete

Actually I've just thought, in the first case it could be translated literally as:
"This is the time of the lunch",  i.e. déjeuner is a noun not a verb, unlike dormir.

(Should have gone to Specsavers berfore posting!)
 

Not an expert BUT, du may be used in front of a masculine noun in this case.