So I have spent about 12,500 marks on sleep resist to see how it works. As it already states when you mouse over the sleep resist it says shortens duration. I qued into my friends in 4s and tested 5 sleep resist (just food HQ) tested then 10 sleep resist (5 melded sleep resist) and 15 sleep resist (melded and food HQ).

NO resist : First sleep last 30 seconds then 15 then 8
05 resist : First sleep lasts 28 seconds then 14 then 7
10 resist : first sleep lasts 27 seconds then 13 then 6
15 resist : first sleep last 25 seconds then 12 then 6

With all this data I found that the Number of resist you have is the % of the durations lowered. So with that if i had 22 resist i would take away 22% of the duration of each sleep that is the baseline of it.