BAKERSFIELD, Calif. -- There is no question that the human brain functions best when well-rested, but new research has found that we can even strengthen our memories during sleep.

Scientists at Northwestern University gave volunteers a memory test that involved placing 50 pictures into their correct places on a computer screen. Each picture had a specific sound attached -- meow for a cat, a whistle for a tea kettle, and so forth.

After initially learning the task, the volunteers took a nap. For half the group, researchers played the sounds for some of the images used in the memory test.

When they awoke, the participants who heard the sounds while sleeping improved their memory for those items. A control group that did not sleep showed no improvement.

The volunteers had no recollection of the sounds they heard in their sleep, proving that the memory benefits were entirely unconscious.

Experts said that picking up new skills in a person's sleep may not be possible, but subliminal learning is possible to enhance information that is already there.