4. Loss of Coordination/Muscle Rigidity
Serotonin converts to melatonin the sleep hormone, telling the body that it should enter a state called ‘rest and digest’. This is the opposite of ‘fight or flight’ and occurs when the body is safe and has an abundance of raw materials (nutrients and energy). It can that way than direct resources toward restoring tissue, digesting food, strengthening the immune system, etc. rather than driving muscles and brainpower. Muscle relaxation helps to conserve energy, allow blood flow, and also leads to sleep.
Thus, it follows that an imbalance in serotonin can also lead to an imbalance in muscle control. Some people experience this as rigidity as the muscles become tense, while others will experience it as a loss of coordination.