The absence or loss of muscle tone, which leads to a decrease in muscle tension and a decrease in resistance to passive movements, can affect individual muscles or the entire body and is often observed during sleep, especially in the rapid eye movement (REM) sleep phase, when muscle tone is temporarily suppressed to prevent the performance of activities during sleep. Atony can also occur with neurological disorders such as brainstem lesions, motor neuron disease, or neuromuscular diseases such as myasthenia gravis.