"relentless night" is correct and usable in written English.
You can use it to describe a night that seems to drag on forever and doesn't provide any respite or relief. For example: "The endless thunder and lightning of the relentless night ensured the entire family was kept awake all night long."
But the city's flaws — high prices, street crime, incessant drizzle — are no match for its strengths — artistic and business energy, relentless night life.
In a relentless night battle in Louis Armstrong Stadium, No. 8 Fernando Verdasco came back from two sets down to oust No. 10 David Ferrer, 5-7, 6-3 (8), 6-3, 6-3, 7-6 (4).
They were relentless tonight".
At our butchershop the radio played while we stood in line for a pound of beef heart; our taxidriver was similarly relentless this morning.
The men stand stripped to the waist, in their thousands, against the wire in the relentless afternoon heat; the women and children seek shade upstairs in the crowded, smelly building.
The men stand stripped to the waist, in their thousands, against the wire in the relentless afternoon heat the women and children seek shade upstairs in the crowded, smelly building.
I thought we were pretty relentless tonight.
Shopping centres, with their huge generators, are about the only place you can bank on air conditioning in the relentless midday heat.
I thought we were pretty relentless tonight". Chances continued to come and go for Quins with the Saracens pack keeping out everything thrown at them, while being particularly impressive at the breakdown.
In hot climates, it is best to have some shade from the relentless afternoon sun, so a flower bed to the east of a building or fence usually works best.
Butler smothered Rashad Phillips, the league's most valuable player, and his Detroit teammates with a relentless defense last night, routing the Titans, 62-43, to win the Midwestern Collegiate Conference tournament title in Chicago.
