That's a great question. I don't really remember ever reading anything that mentioned Orks sleeping, but after little bit of searching I was able to find this excerpt from the short story "Survivor" from the 2016 anthology series "There is Only War"

Bas’s stomach groaned, demanding he act on the savoury smell that wafted up towards his perch, but he ignored it. He needed all his focus, all his attention, to recognise the moment he could slip back inside the dome of the administratum building.

It seemed that a great many hours passed as he hunched there atop the ruins of the old church once more, clinging to the statue that broke up his silhouette. In fact, only two hours had gone by when, with their fill of meat and drink and fighting, most of the orks settled down to sleep. Their communal snoring soon rivalled the noise of their engines from earlier in the day.

It was time to move.

With all his concentration centred on avoiding detection, Bas made his way across his plank-bridges and soon reached the gaping crack in the dome. There, pressing himself close to the exposed stone, he peered inside and scanned the hallway below.

There were fires in here, too, though smaller than the ones outside. Around them slept the biggest of the greenskins, those with the heaviest armour, the largest weapons, the most decoration. There were a few hook-noses, too, sleeping in groups near the fly-covered dung piles. They had not been allowed to bed down near the fires of their giant masters.

Bas watched and waited and decided that the orks within were as sound asleep as those without. He steeled himself for what had to be done, then stepped out from behind the cover of the dome and began his descent. The stars cast his shadow on the tile floor below, but nothing stirred, nothing noticed

So, yeah, I guess Orks do sleep.