Somnolence is a sleepy state. New parents often stagger through their days in somnolence, after spending their nights up with a baby who only sleeps for a few hours at a time.
Use the noun somnolence when you're talking about drowsiness. You might be overcome with somnolence in a boring class, especially if you didn't get much sleep the night before. Also, someone who seems to be out of it, going through life like a sleepwalker can be described as experiencing somnolence, which comes from the Latin word somnolentia, sleepiness.
