During naps, most sleep is in the deepest stages, characterized by slow, regular brain waves, Dr. Dinges said.

But it can appear as a side effect of sleep medications that increase the time spent in the deepest stages of sleep.

In fact, it occurs when the sleeper is enjoying his most oblivious, deepest sleep-a stage in which dreams are not usually reported.

Many experts believe that the deepest stages of sleep, known as "slow-wave" sleep, the times when you are hardest to wake, are important for producing a well-rested feeling.

Our deepest stage of sleep, known as delta sleep, is the time when the body's growth hormones kick-start the repair of cells and tissues.

Surprising Evan in his deepest stage of sleep might make things go smoother.

He seemed in the deepest stages of shock, his eyes sunk in their sockets, unfocused.

In older children and adults, it tends to appear during drowsy, meditative, or sleeping states, but not during the deepest stages of sleep.

They may repeat this cycle an exhausting 600 to 800 times a night without ever getting into the deepest stage of sleep but may remember almost nothing.

While nightmares usually occur in the same stage of sleep as other dreams, night terrors occur in the deepest stage, when dreams typically do not.