Page 54: SLEEP DEPRIVATION. I feel this is quite lame, actually. I'm 67yrs old. All of my life I've been able to easily go 24hours before needing sleep (though I've had to adapt to a 'normal cycle due to operations reasons) and 30 with little difficulty. Getting to 36hours and I'd start to have major difficulty. True, this is real life (and I'm not longer quite as able).
Requiring sleep to function after 18 hours is ridiculous. Perhaps if you go more than 2 cycles of 4/1 work/rest shifts, or more than 5 shifts without a shift of rest the difficulties arise, though this should be able to be put off by at least 1 shift by a stretch of rest. Requiring a 3/1 work/rest cycle without penalties is not reasonable.
All that happens if you don't sleep is that you loose your capacity to recover WP and conditions and d6 WP per shift after the three. That is not so scary.
To loose d6 WP is bad but represent that you loosing your focus a bit, it is 3-4 on the average. The lose the ability to recover WP isnot that scary either because you can only recover HP/WP while doing a short rest once per shift, so that is not so much loss (you can still recover HP).
So, most characters will be able to go a couple of shifts over the limit if they must with not so much loss.