NEW YORK (CBSNewYork) — You’re exhausted, drained, running on fumes.

But how do you know if you’re really sleep deprived?

As CBS 2’s Alice Gainer reported, a nap can actually provide the answer and reveal what your body needs.

When you get a good night’s sleep, dreaming typically occurs during the fifth stage of sleep, called rapid eye movement, or REM.

If a quick nap leads to dreaming, sleep experts say you’re so sleep deprived your body actually skips the first four stages of the cycle.

“If you’re dreaming during a nap that means you’re not getting enough sleep overall,” said Dr. Rafael Pelayo, a clinical professor at the Stanford University Sleep Center.

When you’re not getting enough sleep overall, it can lead to a surprising number of health problems, from heart disease and stroke to diabetes.

New research from the University of Pennsylvania shows a daily nap may reverse some of the potential damage.

“It can help you compensate for a lifestyle of reduced sleep at night … ,” said David Dinges, who heads the division of sleep and chronobiology at Penn. “Make your alertness better. It can improve your memory.”

From dogs and cats, to horses and elephants, virtually all mammals benefit from naps.

But doctors say most humans do not nap.

“You have to understand that you’re taking a dose of sleep,’ Dinges explained. “You have to think about it the way you might consume a dose of food, and then you don’t want to overeat, and you don’t want to undereat.”

Dinges said the amount of time you spend asleep is key.

To increase alertness, he suggested a 10- to 20-minute power nap. A daily hour-long nap can help improve your memory. But Dinges warned that the longer you nap, the groggier you will be when you wake up.

Another sign you’re sleep deprived is if it takes less than five minutes to fall asleep at night. It should take between 10 and 15 minutes.

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