Engage in bracketing. This is a tip that comes from conflict management, and it carries over well to research and writing. When we are engaged in conflict, many different issues and concerns might come to the fore. If those issues create a digression, the original source of conflict might get lost and might never get resolved. Instead, the effort toward resolution turns into an even muddier puddle. In managing conflict, you should be mindful of those side issues and point out that they should be bracketed for a later conversation, once the current conflict is resolved.
As interested, engaged researchers and writers, we often find side interests that might take us down rabbit holes. This happens even after you have stopped reading your email and shut down your internet connection. Keep a notepad or an open document where you can jot down the “save for later” topics and keep yourself on track.