Asceticism
This is the practice of voluntarily denying pleasure or even basic human needs, usually in order to achieve some higher spiritual awareness. This is not confined to a single religion. Ascetic practices such as fasting are common in many belief systems.
A practitioner is called an “ascetic.”
The name comes from the Greek word for “practice” or “exercise.” It’s pronounced “uh-SET-ick.”
Why I Looked It Up
I started seeing an image on Reddit of an Indian (presumably Hindu?) ascetic with a metal grate welded around his neck in order to prevent him from ever laying down horizontally.
I had always confused the word with “aesthetic.”