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?