Watering lawn at night prevents evaporation and keeps the grass happy and healthy.
Watering lawn at night exposes it to fungal pathogens ( an orangish rust growth over the grass). Fungal pathogens love cool moisture and dark places. Watering your lawn late in the day and/or at dusk gives them exactly what they need to thrive. So stop it! Water your lawn early in the morning; this gives the grass blades time to dry out during the day, while minimizing water evaporation from the soil itself.