"He sleeps an hour at night."
Translation:Han sover en time om natten.
Jesper wulfe had given a really good explanation of this earlier for a question regarding why it was sondagen and not sondage.
This is apparently similar to the expression ' i drink coffee in the morning'
So though it is the definitive it actually means every night. He sleeps an hour in the night ( as a rule or habit) Hope this helps.