I found this model years ago (I think it was modified from AD&D Wilderness Survival Guide)and have used it since very effectively - to the chagrin of my players, who must now factors their character sleep requirments into their nighttime sentry duty. SLEEP
Every character requires at least 1d4+4 hours (fixed) of sleep every 24 hours (need not be consecutive), but no more than 2 hours beyond that minimum can be slept. It is possible to train oneself to go with less sleep, and the minimum hours required can drop by up to 20%.
A special Sleeplessness Universal Penalty
is imposed on all MLs according to how many hours less than the minimum required per 24 hours were slept that day/night (Fractions rounded up):
2 HOURS LESS: -5 ML
4 HOURS LESS: -10 ML
6 HOURS LESS: -15 ML
8 HOURS LESS: -20 ML
10 HOURS LESS: -25 ML
12 HOURS LESS: -30 ML
The above fatigue is special in that it cannot be negated until the character sleeps fitfully for the minimum hours. Note also that the values are NOT cumulative. If, say, on day 1 a PC who needs to sleep for 7 hours sleeps for 3 hours, he will suffer a -10 ML penalty. If, on day 2 the PC only sleeps for 1 hour, he will suffer a -15 ML penalty the next day. If on day 3, the PC sleeps for 6 hours, the next day he will only suffer -5 ML penalty.
The sleep must be good. This is defined as:
THE NUMBER OF HOURS SLEPT,
-1 HR, IF CHARACTER NOT PRONE OR IN RELAXED POSITION.
-1 HR, IF LYING ON BARE GROUND (NO BLANKET/PADDING).
-1 HR, PER 1-IL SUFFERED IN WORST INJURY.
-1 HR, IF FREEZING TEMPERATURE.
-2 HRS, IF HOT TEMPERATURE.
-2 HRS, IF WINDFORCE IS 2+ WITH NO SHELTER.
-3 HRS, IF PRECIPITATION WITH NO SHELTER.
-3 HRS, IF WEARING RIGID OR METAL BODY ARMOUR, OR
-4 HRS, IF WEARING PLATE BODY ARMOUR.
These adjustments are cumulative (except the last two), and determine the hours effectively slept by a character.
No character can sleep without shelter in severe weather conditions (storms of any sort).