How to overcome fluctuating productivity

Problem

Hello DEV-Community i have a problem. I work now since nearly 3 years as a Data Scientist with a health insurance company. Most tasks are usually easy to accomplish.

However, I notice that the longer a task takes, the less motivation I have (according to Robert C. Martin, Clean Code [1]) and my productivity fluctuates daily.

[1] Robert C. Martin, Clean Code, Page 37, 4A

There are days when I get a lot done and others when I find it difficult to motivate myself. I notice that this happens to me even when I have deadlines and I don't want to miss a deadline.

Question

I think that many people have this problem and that fluctuations are normal and are of course determined by many factors (sleep, diet, stress, etc.).

However, I wanted to ask how you deal with it and how you deal with longer tasks and the decrease in productivity.

Do you use special software or special working methods or how do you keep your productivity at a certain level.

Further Action

If there is no thread on this issue yet, I would like to collect the answers and create a HowTo or a Github repository from them.

I would also be happy to receive book suggestions that might help. I think this topic can help a lot of people, especially CodeNewbies.