When you're lying around being lazy, you're in a state of inactivity — you're not doing anything. Some animals, like cats, typically have hours of inactivity every day.
Your inactivity might be a symptom of boredom or exhaustion, since it's characterized by slowness, sluggishness, and a general lack of energy and motion. Inactivity in the middle of the night is perfectly normal, but your daytime inactivity will make it difficult to get anything done. The noun inactivity adds the prefix in-, "not" or "opposite of" to activity, which comes from a Latin root, actus, "a doing, a driving, or an impulse."