I used to wash my hair in the morning, but it was added hassle when I don't have much time to get ready. There was also the problem of drying it. Generally I don't like diffusing, and if I do, I can't diffuse too much at all, have to keep adding product etc, so it's very time consuming. If I leave it to air dry, it takes a good six hours to completely dry, maybe more. Also, I don't know how it will turn out and can't fix it when I'm out and about.
Washing at night means that my hair is bone dry when I wake up. All I have to do is scrunch the crunch a bit. I can see exactly what the 'finished product' looks like, and I can always make alterations then. Also, because my hair is on the pillow, it curls up more. Lastly, because I'm indoors and asleep, I'm not touching my hair, and it's not being affected by weather or anything.