This should stress the point- don't expect birds to sleep at night. In fact, diurnal species often vocalize during the night, especially when disturbed. No sounds have caused as much confusion as owl sounds. This is partially due to some well known researchers' spreading the word that Bigfoot mimics owls. Owls can make a tremendous variety of surprising sounds.
Most people are only aware of the more common calls.
Since most owl calls occur during darkness, it can be difficult to obtain positive identifications of the source. This should stress the point- know your bird sounds, and never underestimate owls. Here are clips of some of the more confusing owl sounds. This is a young Great Horned owl. These calls were heard almost every night for 7 months in their home territory.