Actually it's surprisingly not stressful--one interesting thing I've learned is that, at least for myself, job stress and burnout comes from something not going that well (e.g. my old startup), not from being super busy.
And to the parent comment--I'm fine, thanks for the concern :)
That sounds right. When things weren't going well at the startups I worked at I felt crazy stressed out.
I also felt some stress when busy, but that was less because there was a lot to do and more because I had someone who was expecting it to be done in an unreasonable amount of time.
I have had two periods of great work stress times in my life - the first was when my startup was under enormous financial strain and I was doing everything I could to hustle up the money to keep the team together (2002 to 2003). This was a time of lying awake night after night wondering what to do.
The second time was different. I was working two more than full time jobs (academic and running startup that was taking off). I literally did nothing for a couple of years (2010 - 2012) but work, eat and sleep. At the time I didn't feel stressed (I was comparing how I felt with the early stress times), but I was under a lot of stress. I only realised when I quit my academic job.
And to the parent comment--I'm fine, thanks for the concern :)