I did 36-hour days (awake for 24, asleep for 12) for a long time and it was fine. I'm unable to do so now, due to other obligations, but I still think it worked better for me than a 24-hour day. Even today I inevitably stay up all night, getting at most 1-4 hours of sleep, 2-3 times per week.
I don't see anything problematic with TFA's 28-hour days.
I lived on 36 hour days for a good year or so as well. It made being social very difficult, and was not the greatest thing for me overall, although I did get a lot of code written.
A few months ago I finally got a CPAP machine after getting a sleep study due to abnormal blood pressure patterns (my blood pressure wasn't dropping overnight on a 24 hour monitor). I can now wake up feeling alert and ready to go after 6 hours of sleep (even though I could use a little bit more); I haven't felt rested this way in the morning since more than 15 years ago in my 20s. I'm not particularly overweight, nor do I snore, so it's worth checking into if your sleep patterns tend towards abnormal.
I think I'm somewhere on the long right tail of needing a lot of sleep, I feel best with 10 but I can do with 8. Less than 8 and I don't feel too good.
In the winter I can do 12 hours pretty often. It's not always but every few years I get a few weeks or months of 12 hour nights.
Not who you are asking, but for me if there's no noise or activity in the house, I usually just keep on sleeping. 10 hours would be more common for me but I've done 12 hours pretty easily too.
I don't see anything problematic with TFA's 28-hour days.