Why do Spiritual people try to sleep less? In Monasteries and other spiritual contexts, there seems to be a "rule" that sleep is wasted time, and so the usual daily schedule only allows for about 4-5 hours of sleep. This might be enough for an 80-year-old, but anyone else will probably be sleep-deprived and not very functional, let alone be able to make any spiritual progress.

I asked about whether adults can get by with less sleep, and got no answers. Does anyone know why spiritual scenarios seem to try to reduce sleep hours? One thought is that people think it is associated with laziness, indolence, lethargy, unproductive time, idleness, sloth, etc. But no one tries to reduce food intake, or use of the toilet, so why is sleep the big bad guy?
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".