When we get cold, our body shivers and spends energy to raise our temperature. When we get hot, our body sweats and spends energy to lower our temperature. The human body prefers a particular temperature and tries to maintain it against however hot or cold it's surroundings may be. It prefers other things, too, such as a certain PH balance. This preference and regulation is called homeostasis.
When the body finds itself doing the same thing over and over, it takes note: "It is always cold." It then adjusts: "Since it is always cold, I am going to be ready for cold." The body expects cold surroundings and doesn't waste letting its temperature fall too far. The change in the body's expectations - its standard for homeostasis - is called allostasis.
Sleep deprivation can cause an allostatic shift for your body. If you are regularly getting less than 8 hours of sleep each night, your body adjusts to this and makes the best out of the 6 hours you may be getting. This doesn't mean 6 hours of sleep is now healthy for you. If you were undernourished, your body would slow metabolism to stay alive. Allostasis is about being as effective an organism as possible given the circumstances. It doesn’t take into account how it feels to be that organism.
Our brains try to compensate: “This is the new normal, so I’m going to get used to it.” Getting used to things is just how we’re wired, since it has helped us survive incredibly harsh, changing environments. Problems crop up when there’s pressure to have a different allostasis: life expects you to operate as though you’re getting a solid 8 hours of energizing rest. “How much can you give” is expected in all venues of living - at work, in family, at school, and volunteering. A war of expectations emerges between your self - your body’s energy - and your environment.
Protecting yourself takes courage. It is not easy to erect a boundary between the outside pressures of life and one’s needs; bizarrely, taking care of yourself in the most basic way is considered selfish. Building this boundary is an investment that takes time, and rewards in time.