The shepherds that were visited by the Angel of the Lord were keeping watch over their flocks by night. At a time when they could have chosen to be asleep, they chose to be awake and on guard. One major reason why shepherds keep watch over their flocks is to guard against any invasion or attack that would lead to a loss of their flock; so those shepherds knew that they had people waiting to steal from them, and they were on guard.
Friend, as you run your race in destiny, be aware that there are people hanging around seeking for a chance to steal from you; there are powers of darkness seeking a chance to reduce your destiny; there are wicked men and women who have aligned with satanic powers, seeking to spoil your success; there are enemies of destiny whose agenda for you is to make you go down so that they will go up. There are men waiting for you to sleep so that they can prey on your destiny.
Always be on guard. Guard your destiny with prayers; guard your family with prayers; guard your business, career, academics, marriage, children, etc with prayers. If there be any door of success that God has opened unto you, guard it with prayers, lest you wake up one day to find the same door closed for no reasons. If there be any opportunity that heaven has given you, guard it with intense and continuous prayers.
The shepherds seemed to understand that those who would want to steal from them would come in the night, when most men would have slept off. They knew that their low moments could make them vulnerable. The enemy seeks to take advantage of your low moments to make a mess of your destiny. In moments when you are tired, remember that your adversary the devil is not tired.
Arise, wage war in the place of prayer, scuttle the plans of the enemy, and stop the enemy from making an inroad into your destiny.