[hintymad]

I'm guessing for someone learning a new language is relaxing and therefore helps recharging the person after hours of intense PhD work - things like enjoying daily progress, discovery of foreign culture, the euphoria of being able to read and watch new stuff...

[latentsea]

Learning a new language is a stressful grind. I've studied CJK at a similar pace as this article, and it's equal parts exhaustion as it is elation. Not for the feint of heart.

[fn-mote]

Exactly this.

Unlike the PhD, they make daily progress on the language. Success is visible.

(Edit: ok, the truth is they were not doing this at the same time as the PhD. I still like my comment.)