Albert Barnes Commentary Acts 12:7

"And behold, an angel of the Lord stood by him, and a light shined in the cell: and he smote Peter on the side, and awoke him, saying, Rise up quickly. And his chains fell off from his hands." — Acts 12:7 (ASV)

And, behold, the angel of the Lord. See Barnes on Acts 5:19.

Came upon him. Greek, was present with him; stood near him, epestē.

And a light shined in the prison. Many have supposed that this was lightning. But light, and splendour, and shining apparel, are commonly represented as the accompaniments of the heavenly beings when they visit the earth (Luke 2:9; Luke 24:4). Compare Mark 9:3. It is highly probable that this light was discerned only by Peter, and it would be to him an undoubted proof of the Divine interposition on his behalf.

And he smote Peter on the side. This was doubtless a gentle blow or stroke to arouse him from sleep.

And his chains, etc. This could have been only by Divine power. No natural means were used, or could have been used without arousing the guard. It is a sublime expression of the ease with which God can deliver from danger and rescue his friends. Compare Acts 16:26.