Sorry, I know I am bringing up an old topic. But new people might look this up so I wanted to add something.
First of all, like the others have said, it's a bad idea to water at night. And like HooKoo said, it's mainly because it promotes disease and insects. Specifically, watering at night isn't the problem. The problem is watering on WARM SUMMER NIGHTS! When it's above 60-70 degrees at night, and you add water to the mix, now you have the perfect breeding ground for insects and disease.
Watering early morning is best. Typically 6am-9am depending on how hot it is. If it is really hot in your climate, might want to start closer to 6.
I have seen people in Tx run them at night without issues. I have seen turf comps do at night and be fine. I myself run at 4 am but wonder about this debate.
You can inject ferts into the system so I do not buy the only in the mornings.