If you’re working full-time and you want to write a book, your biggest challenge may be carving out time to write. You probably don’t have blocks of time lying around.
You’ve got social and family obligations, not to mention demanding work. Your calendar is already full. You must take time from something else to have time to write your book.
Sleep less.
It’s a bad idea. You will probably need more sleep, not less.
Reduce time with friends and loved ones.
Be careful. This can look good until it suddenly goes bad. The people who love you will support the book project; they’ll put up with a lot and probably won’t say much until they’ve had as much as they can tolerate.
Reduce time spent on hobbies and other activities you enjoy.
You can probably get some time here, but not a lot. Those hobbies and activities are part of your recovery time. Many are also social events.
Work less.
This can work, especially if the folks you work with will benefit from the book. If you work for yourself, working less will probably mean less income.
It will be stressful no matter what you choose.