To the fun theory, hedonic treadmill sequences.
http://gettingstronger.org/hormesis/
TL;DR stoicism with science.
Key idea: OPT, Opponent Process Theory: http://gettingstronger.org/2010/05/opponent-process-theory/
Research, PDF: http://gettingstronger.org/wp-content/uploads/2010/04/Solomon-Opponent-Process-1980.pdf
From the article:
"In hedonic reversal, a stimulus that initially causes a pleasant or unpleasant response does not just dissipate or fade away, as Irvine describes, but rather the initial feeling leads to an opposite secondary emotion or sensation. Remarkably, the secondary reaction is often deeper or longer lasting than the initial reaction. And what is more, when the stimulus is repeated many times, the initial response becomes weaker and the secondary response becomes stronger and lasts longer."
I think it's complicated-- what kinds of challenge are good for people, and how do you judge how much challenge of a particular type is good, and for whom?
So far as I know, being low on sleep is bad for people. Um, except that sometimes modest amounts of sleep deprivation (like an all-nighter, I think) can raise energy. Again, vague impression, but some missed sleep can work as a useful reset for some people with depression, or set off mania for people with bipolar.