Most people never think twice about waking up with a damp pillow. They simply wipe it away and continue their day, unaware that nighttime drooling can sometimes reveal surprising clues about what happens inside the body while they sleep. Although it is often completely harmless, experts say it may reflect how deeply the brain relaxes during certain stages of rest.

One evening, Daniel laughed when his wife pointed out the small wet spot on his pillow again. Curious, he searched for answers before bed. The information he found seemed ordinary at first, until he noticed one sentence that made him sit upright…