John Gardner in his On Becoming a Novelist (highly recommended) says something about writing first draft at night, when the mind is more dreamlike and revising in the morning, when the mind is more clear and logical. Similar advice has been mentioned in the interviews which form part of the materials for the creative writing course I tutor.
So, tired or not, I sat down at the keyboard. First time I did a word count, it was just over 100 words. I tappety-tapped a bit more and counted again and it was something like 280. Oh, I can write a few more and make it 300, I thought. More typing. Pause again for a word count. Now it's nearly 400. I'll just do 400 and stop. 400 words a day is my target, so it will be good to meet my target. But then the words came a bit easier and some rapid typing brought me to the end of the short scene and a grand total of exactly 600 words. And when I looked at how many words there were in the chapter, I decided that was long enough and the scene end is a good stopping point. So chapter 9 is now finished.
So it works. It seems I can write even when tired, as long as I have something pre-planned so I know where the scene has to go and what it's supposed to do.
Words today: 600
Words in Chapter 9 (now finished): 3661
Words total: 44,017
Reason for stopping: End of scene and chapter.
Other: Fortified by Russian Caravan tea.
But I must have an early night tonight to catch up on some sleep before I have a long day of travelling on Thursday.