"deep night" is a correct and usable part of a sentence in written English.
For example, you could use it in a sentence like this: "The sound of crickets filled the deep night air."
In deep night, if you are extremely lucky (and quiet), that guard will be asleep.
Even the most secure houses achieve, in deep night, a state of relative unconsciousness.
Lynn Saville has wandered purposefully through every phase of the nocturnal world, from dusk through deep night to dawn.
In some paintings her palette centers on muted, desaturated earth tones; in others it exploits saturated colors from deep night blues to vibrant oranges.
And what if future astronomers have no such records, perhaps because on their planet scientific acumen developed long after the deep night sky faded to black?
The mooncake, a bright, warm, golden confection against the backdrop of deep night and stars, entices Little Star, who nibbles on it a bit every night.
We had to dig deep tonight".
Meanwhile, in the Nazi-occupied Europe that the literary superstar had fled, total war had reached its deep midnight.
Sosa and Mark McGwire both went deep last night to remain neck and neck in the home run chase.
John Varvatos makes dinner jackets with narrow lapels in a deep midnight blue or chocolate brown (left, $895 at John Varvatos stores).
One sublime pleasure was taking a deep afternoon nap on an incredibly smooth, flat rock in Doctors Cove in La Manche Provincial Park.
