In fact I could be that other engineer (I'm not, but I'm in a very similar sounding situation)
I was the most senior engineer and first to join a new startup with a bunch of colleagues from an old work place. I put in ridiculously long hours for the first year and a bit to get the product started, and got completely burnt out and my family life suffered
After we brought on a few more engineers I reduced my hours to a more normal 40 hours a week, and made use of the flexible start-up hours to pick up my kids from school some days and come in late on others (A great way to manage the stress of being in a busy startup)
Now I also spend a lot less time writing code and more time managing product requirements, an overseas team, on system architecture, and all those dev-ops tasks (big time-sink) that no-one else wants to do
I don't completely know the original-poster's situation, but quite often the senior person on the team is often shielding the other developers from all the daily crap that is going on in a company so that they can be as productive as possible, and you might not be completely aware about everything that is going on