But to answer your question, I don't think it's just the feeling of finishing a task. Yesterday, I woke up and felt well rested. I stretched, made breakfast and had some introspection time, started grabbing tasks one at a time from my side project and finishing them. Then I picked up a book that I've been reading and read for 2 hours and I also finished some random chores.
This morning, I woke up and I cant get out of the bed (this is not just Monday effect, I've felted on other days). I don't know if this exhaustion is because of being too excited last night from all I've done and not being able to have a good night sleep, or just I've exhausted myself yesterday and don't feel like doing anything today. Or I would have felt this way anyways.
I do think though the ground work is done on the less productive days in terms of restoration. My girlfriend got a dog about 10 months ago, and I can see the same behavior in the dog. She runs around and exhausts herself on a hike one day and then just sunbathes and chills the next day. It seems like our hunter gatherer ancestors behaved the same way.
Of course it’s easier to get up for a task that’s exciting, but I found that my motivation seems to be strongly correlated with my physical well being. One part I’ve been looking at recently is what I eat and how that affects my gut health which affects my mood and motivation.
That being said physical wellness is definitely the only factor as I still get quite big down time (had one yesterday) but not as often and long as I used to have.
For two weeks, keep track of every minute of your working time. Once you know where your time went, you can then try to understand why your less-productive days are less productive. Was it because you were too tired to concentrate? Did your “super productive” day involve 12 hours of focused work? You might find there’s not as big a difference as you think between different days, except that some days feel productive because you happen to finish some task, when in reality the groundwork for finishing it was laid on a day that felt unproductive.