personal anecdote.

* i'm a software developer in central europe.
* i suffer from delayed sleep cycle (i.e. i'm a night owl).
* i picked up weightlifting (as the main hobby) to compensate for all the sitting and the club has fixed workout times in the evening (i.e. no 24/7 gym access). the harder i work out, the more sleep i need.

i used to work a 42 hour work week (from 7:30am - getting up at ~6:30am to ~6pm) at a small web dev company in the countryside for 2 years. close to the end i was practically burnt out, close to tears every evening and broke out in celebration when they told me i was fired. even after 2 years my sleep cylce hasn't normalized - i've been constantly sleep deprived.

afterwards i got another software dev job at a company in the capital where i first worked 25 hours and now work 30 hours a week with flexible time - roughly 6 hours a day monday to friday, no core time (except for scheduled meetings).

i get paid more for those 30 hours than for the 42 hours in the first job, but have less money at the end of the month because i used to live with my parents at the first company.

there are a lot of differences (more competent superiors etc), but personal observation regarding work culture and time:

* i'm usually well rested: due to flexible time i rarely set an alarm and wake up on my own depending on when i went to bed and how much i worked out (or boozed) the previous day. this means sometimes i'm in the office at 3pm. sometimes 7am. doesn't really matter as long as i don't have any meetings (which are very rare; currently once a week at 2pm). if i want to stay home for the day i send my teammates a message.

* stress level is extremely low because i can always take time off without much bureaucracy if i have to attend personal matters.

* due to only working 6 hours a day i can easily accumulate overtime which isn't paid but goes into an 1:1 overtime pool. this enables the next point:

* i rarely sit idle. if there isn't any work to do, i'll go home. period. if there's a lot of work to do (and i'm in the zone), i'll happily stay longer. it evens out and i never idly wait for the clock, passing time. i usually work more during the winter and less during the summer, when customers are on vacation.

* if i'm feeling unwell but not sick (or not sick enough for sick leave) i work from home and take frequent naps. if i'm sick it's rarely for long because coming to work sick is heavily discouraged and i just stay in bed. i go back to work as soon as i feel healthy and ready even when my doctor approved more sick leave.

* if my concentration is slipping and i don't get any work done, i call it a day. if i'm super productive and in the zone i stay because i want to.

* because of all those things i love working there. i think my employer is fair, understands efficiency and treats us employees well and i wouldn't want to cheat the company because "do as you'd be done by". i wouldn't call in sick if i had a hangover (or are otherwise unfit to work due to my own fault). if i work from home i don't do the dishes on company time. i'm honest - and so is my employer (as far as i can tell - i never caught them trying to cheat me). loyality is high; in case i suspect a colleague of cheating i'm actually on my employers side and tell them to quit their bullshit because of self interest (i don't want to lose those perks).

* i don't think i could deliver the same bang for the buck with an 8 hour work day because i wouldn't be able to accumulate the overtime i'd need to take time off if i'm not at my best. but if i wanted more - or even less - that'd be possible.

* i still earn enough to get by comfortably.

* i don't have kids yet but many of my colleages do and they love working part time because it makes things so much easier and enables them to spend more time with their offspring, even though they earn less. but this way their spouses can work more and earn more, so it evens out - and they can share responsibilities easier.

TL;DR: in my opinion 30h/week is the best bang for the buck for both me and my employer because i only work when i'm productive, so the company doesn't have to pay for my idle time and i can minimize stress and frustration. i understand this is partially possible due to the nature of software projects (deliver on time, but it doesn't matter much when you do the work).

edit: also, i can do 6 hours without the mandatory 30 minute break, if i'm in a hurry.

edit 2: i can't offload work to teammates. if i take time off, work just accumulates. no cheating!