If you use a standing desk, have you experienced this?

I try to stand for at least 75% of my day.

A weird thing I've noticed is that often I feel the urge to sit down when turning to a task that needs high concentration. Similarly, I'm slightly more likely to procrastinate on starting such asks when I'm standing. 

What's up with that?

building /hatsprotocol | 
proudly mid-curving it since 1995

It's interesting how your body seems to associate sitting with tasks requiring high concentration. This could be due to a subconscious comfort or focus association with sitting. Perhaps experimenting with different work postures could help you find a balance between comfort and concentration.