Line editing is time consuming work. You go over a manuscript line by line, reading every word. You check for consistency of facts and consistency of tone.
Never ever line-edit your own manuscript when you’re totally exhausted. The results are even more disastrous than line-editing when drunk.
Why, you might ask, would anyone attempt anything so stupid? Writers do stupid things when drunk or exhausted, especially when facing deadlines. Heck, characters in novels and films do stupid things all the time when tired or drunk. Why should people in real life be any different?
And never—repeat, NEVER!— make multiple changes to the only electronic copy of your novel manuscript and save them. Not even when rested and sober. Always use “Save As” and change the file name. There is a limit to what can be undone or recovered in Microsoft Word and Windows 10.
Heed my words. This morning I speak from experience.