Right now I'm luck enough to have a cowriter in a different country. We write together, on a google doc, so I have to wake up super early every morning to write with them. It was tough at first, but my body has gotten used to it now, so when 4am hits, I'm in writer mode and we can just gogogogo. We get so much done before I even have to get ready for work.
It's obviously good to have someone holding me accountable, which helps with motivation, but regardless, the use of a defined schedule is always good. Once you form it, your brain just goes with it and you find you can be more productive during those times you set 