Sleep can refer to laziness and Jesus warns all to be alert. Watchfulness was not lacking and the blame for the mixed field went not to the servants, but to the insight of the enemy choosing to do his work of sowing the wicked among the good in the darkness of night. The servants were not neglectful but the enemy was stealthy and malicious. Satan sowed seed accompanying the Children of The Kingdom of God, the good seed, the wheat. The enemy sowed weed among the wheat, the growing plants gradually become identifiable, and the servants tell their master about the weeds.
Wheat is a wholesome grain which ripens by absorbing light, it is good food.
The difference by appearance is that darnel grain has a black spot at it tip. The evidence to the servants showed them that the owner is the Lord Jesus. The owner blames an enemy but the owner forbids his servants from attempting to separate weed from wheat till the harvest. All live together in the world until the end of the age. To the inhabitants of the Judea and Galilee who were Jewish disciples at the time, this came as a stunning proclamation, since they assumed the coming of the Kingdom of God would mean only the destruction of the Gentile wicked.