When they returned from 2015 to 1985....

When they returned from 2015 to 1985, it was night.

At the bit when they first arrived back in 1985, before they discovered that Biff had changed the past, it was dark. After Marty came back to what he thought was his own home, he found a girl in bed. Which implies it's at least late enough for young children to be going to bed.

However, at the start of the movie, when they left for 2015 it was the morning. Marty had slept in to about 10:30am, woken up and been surprised by his family and new car, and then got into the time machine with Doc. That would've been around say 11am, or so.

I wonder why they decided to return to 1985 several hours after they left it.