In response to the above.
A pulled muscle is the result of an over extension of a group of muscles or a strand of muscle within a group of muscles. Sometimes, a twisted muscle can also be “pulled”.
Twisted muscles will cause pain when one uses them. They do not tend to relax by themselves. They tend to remain “twisted”.
Pulled muscles can relax by themselves but self relaxation of pulled muscles may take some time. Pulled muscles, if not twisted, will try to readjust by themselves. However, the “pull” has created a new “position” or “stretch” of the muscle(s) involved, and muscles have a tendency to “return” to their “new stretched” position. This is why people often say: “Muscles have memory”.
At night, when one is relaxed, the muscles tend to relax so as to fall asleep. The pulled muscles also attempt to relax but they “remember” the “newly stretched position” so they tend to “want” to return to that “pulled” position.
Thus, “pulled muscles” tend to tighten up at night.