“Brain-derived neurotrophic factor (BDNF), a major brain protein, is decreased, which may lead to an increased chance of depression,” Weiss says. “Dopamine levels drop, which may stimulate anxiety, depression and other mood/behavior shifts, as well as cause a sense of tiredness and fatigue that reduces the impetus to exercise.”
In other words, the less you exercise, the less you want to exercise. You’ll find it hard to motivate yourself to work out, and if you try, you’ll be more easily winded, and basic exercises will require more energy. That’s how two sedentary weeks turn into three or four, and your physical health really tumbles off a cliff.