I think you need to listen to your body.

Is this training, job training or is it just testing your body to see it you can do the work you used to do ? There is a difference between returning to old skills and training to learn new skills. The later puts far more load on the brain.

What do you mean by 'physical and cognitive with stress response' ?

Is the technical reading to acquire new skills/knowledge or to refresh old skills/knowledge ?

Did you get sleep during normal hours last week ?

It is hard to understand what kind of load and stress you are putting on your body with your cryptic top secret job.