I fly with a Dutch airline, on the A330 now, but used to be on the 737.....and I have to say, I was often quite fatigued. We did 6 day cycles where you would fly an early schedule the first three days, getting up at 03:40 in the morning, doing 2, 3 or 4 flights a day; followed by three days of late schedules, where you would end at 12 at night. Most days were 8 to 12 hour days. After this you would get three days off.
I found the transition from early to late very tiring, as well as the work load. The actual flying isn't very stressfull, just the preperation for each flight, hassle at the airports, loads of de-icing in the winters etc etc.
Also please keep in mind that these six days you sleep in hotels, which is not like your own bed; you always sleep less efficient in a hotel, strange noises, airco's that make a lot of noise, doors banging on the hall-way.
After a while I decided to go on a 80% schedule to get some more days off. As the 6th day often ended late (23:00 or in the night), the first morning of your days off was crap and on the third day of your leave you had to go to bed early as you would have to get up at 4 in the morning......so you actually one only has a day and a half off to do all the things you cant do when you are away...not a lot of weekends off as well, which means with a spouse that also works full-time, you don't see her or the kids a lot.
I am on the 330 doing intercontinental flights these days and the rules are muuuuch better!!! Loads more days off and basically less flights....I fully agree that there needs to be stricter rules for short-haul flights.....
Just my thoughts on it.....