There have been some attempts to fix that, but many of them are cumbersome to track and finicky in the details. I prefer my game rules to be simpler.
Simple Long Term Fatigue SystemGURPS already has several mechanics for fatigue that doesn't go away quickly: starvation causes FP loss that can only recovered by eating extra meals, and FP lost from missing sleep can only be recovered by sleeping. These FP become "lost FP", that can't be recovered by simply sitting around. Since that's what I want, using these mechanics would be good.
Most FP lost should be recoverable with only a short rest. Brief periods of exertion doesn't make one tired or hungry the same way that extended exertion does. There seems to be a threshold for fatigue before it can't be recovered quickly.
With all that in mind, here's the rule:
If a character accumulates more than FP/3 in lost FP, then 1 of those FP is long term fatigue. Long term fatigue counts as both an FP lost to starvation (B426) and to missed sleep (B427). For every extra FP/3 accumulated with more than FP/3 lost, another FP becomes long term fatigue. A character never accumulates long term fatigue as long as they never accumulate more than FP/3 of lost fatigue at one time.
Long term fatigue does not count as starvation FP for characters with Doesn't Eat. It also doesn't count as missed sleep for characters with Doesn't Sleep. Characters with both of those advantages and FP scores do not suffer long term fatigue.
Long term fatigue never applies to FP spent in an Energy Reserve.
CommentaryThis is pretty simple and takes advantage of existing GURPS rules. People who only do brief bursts of effort with plenty of rest aren't affected, but anyone who really pushes himself will end up tired and hungry.
It has the side effect of weakening fatigue based mages. It's very easy for a mage to cast 1 or 2 spells in a row that cause them to accumulate long term fatigue. Losing access to 1-2 FP for a day after every encounter makes resource management much more critical. Given that I'm moving more and more toward Threshold magery for other reasons, I don't consider this a real downside.