Why do Spiritual people try to sleep less?
There's a difference between spiritual people and students trying to learn something. Sleep deprivation is an effective tool of breaking old learned routines - you just cant remember them too well when sleep deprived (Source). Once you learned what you need, sleep deprivation becomes very disruptive.

An example given is performing a novel task, like reciting the months of a calendar year backwards(December - November - October...). Old ingrained pattern of (January-February-March...) gets in the way, and people who are sleep deprived are apparently doing better on a this task than control subjects, because the old pattern is not remembered as well. (Same source) 

There are a number of "programs" and "cults" like Landmark Forum, which attempt to teach/brainwash/indoctrinate you into learning some new outlook on life in a very short period of time. It has been proposed that sleep deprivation is a very big part of this "reprogramming".