From an appearance on his daughters podcast. Discussing his focus while writing the screen play for the movie Rocky.

Daughter: You painted the windows black to focus.

Stallone: Yeah I did paint them black, because I didn’t want to know what time it was. It didn’t matter what time it was—“Oh, it’s time for breakfast”—right away you’ve de-railed yourself. “I’ll have breakfast when I’m hungry. I don’t need to know what time it is”—that kind of thing. So I was trying to eliminate all the excuses, because it’s hard to write and you’re looking for any reason—“please someone call”—that’s why I took the phone out, so you couldn’t be called.

Other daughter: So you were straight-focused, painted black windows.

Stallone: One-hundred percent. Black windows, no phone.

Sylvester Stallone wrote the screen play for Rocky in only three and a half days.