Watering of lawns needs to be undertaken at a time of day when it is least wasteful, least able to cause damage, and in turn is most useful and beneficial to the lawn.
Watering must be conducted in The Morning.
Like us, lawns, and all other plants, spend some of their time awake and other times asleep, and like most of us, lawns will be awake during the day, and asleep at night.
So Why Does This Matter?
During its awake time, the lawn is actively absorbing water from the ground, conducting photosynthesis, creating it's food, and growing.
While asleep, the lawn becomes dormant, and these growth and life habits considerably slow down to an almost complete halt. The lawn will no longer absorb water, produce food, or grow in any but an absolute minimalist manner.
The vascular system of lawns works like a blood stream, pumping nutrients and water from the ground, and distributing them around the plant. The vascular system of lawns is powered by the sun, and as such is active during the day and shuts down at night.
When lawn watering at night, the lawn can make no immediate use of the water it is receiving, so the lawn does not absorb it, and it stays in the ground being pulled down towards the water table and away from the root zone of the lawn.
By morning when the lawn wakes and requires water, much of it has already disappeared, and cannot be used by the lawn at all. This results in an unnecessary waste of water that was never used, unhealthy lawns lacking adequate water, or too much water having to be applied in order that enough is left over in the morning for the lawn to use, and to show adequate health and growth.
Watering at night creates a lingering moist environment which can promote lawn disease such as Spring Dead Spot as well as other fungal infestations which can damage lawns and hinder their appearance.