NEW YORK -- Even in the city that never sleeps, it's common to see New Yorkers doze off on the subway. But New York City's police commissioner is instructing police officers to wake passengers in an effort to keep them safe.

More than half of all subway crime in the past year involved a sleeping victim. But the new plan isn't welcome with some passengers.

"Who does it hurt?" asked John Fernandez, 26, who occasionally catches a quick nap during his 40-minute subway commute. "Sometimes you just want to close your eyes."

The city's rules for the 24-hour subway system do not prohibit someone from sleeping on the train unless they take up more than one seat or cause a disturbance.

But Police Commissioner William Bratton insists that "subways are not for sleeping."

"If you are sleeping on the subway, you make yourself a very easy victim," he said.

A recent series of slashings and stabbings in the city has drawn attention to subway safety.

The idea to nudge passengers awake is not new. Transit officers in 2012 noticed a spike in subway-related thefts and went car-to-car on the late shifts.

New York City Mayor Bill de Blasio backed the new plan.

"Almost six million people take the subway, and there are approximately six or seven crimes committed in the subway each day," he said in a radio interview Thursday. "And most of those are property theft."

Homeless advocates questioned whether the initiative is just a way for police to hassle people who use the warm trains for shelter.

"I've slept on the trains, and they're already targeting homeless people," said Damian Mitchell, a member of Picture the Homeless, an advocacy group. "But now it's about to get a whole lot worse."

Associated Press writer Colleen Long contributed to this report