A New York appellate court has mandated a new trial for Donald White, a convicted murderer, due to an unusual reason—his friend was removed from the courtroom for napping during proceedings.

Indeed, the judges Betsy Barros, Barry Warhit, Lourdes Ventura, and Donna-Marie Golia acknowledge that the substantial evidence supports the guilty verdict.

But they say White was denied his right to a “public trial” because his friend was booted, and never mind that it was only for a day.

White was found guilty of entering a Queens marijuana dealer’s residence and fatally shooting the dealer in the head, assisted by an accomplice wearing a “Scream” Halloween mask.

But the trial judge’s decision to eject the defendant’s snoozing pal was enough for the appellate court to throw out the entire conviction and give the murderer a second chance with a whole new trial.

Maybe the judges think the lower courts don’t have enough cases backed up and prosecutors have way too much time on their hands, so they ordered a do-over for whatever reason they could come up with.

Or maybe it was just a knee-jerk reflex by lefty jurists who look for any excuse not to make perps pay serious consequences for their crimes.

Judges, after all, do have some discretion to kick people out of their courtrooms for bad behavior.

But the ruling makes it clear why so many criminals in New York think crime pays — that no matter what they do, even killing someone, they won’t suffer much, if at all.

In a city like that, we wonder how law-abiding New Yorkers can sleep.