Adverb
In a sound manner.
Source: Webster's dictionaryA mother's arms are made of tenderness and children sleep soundly in them. Victor Hugo
So she loved him. She just did immediately and again often and clearly naturally and soundly and obviously and many others. Daniel Handler
We must never forget why we have, and why we need our military. Our armed forces exist solely to ensure our nation is safe, so that each and every one of us can sleep soundly at night, knowing we have 'guardians at the gate.' Allen West (politician)
Affairs sleep soundly when fortune is present. Greek Proverb
The less you know, the more soundly you sleep. Russian Proverb
The poor sleep soundly. Japanese Proverb