The antonyms for the phrase "sits up" are "lies down", "reclines", "rests", and "snoozes". These words indicate a state of relaxation where the body is not upright or alert. When someone lies down, they typically assume a horizontal position, often in a bed or on a couch. They may recline or prop themselves up with pillows for added comfort. Resting can refer to a deliberate period of inactivity and relaxation, or simply a break from physical activity. Finally, snoozing implies a more profound state of unconsciousness, such as during sleep or a nap.