To stop an overactive sympathetic nervous system you can do some deep breathing exercises, mindful practice techniques and even gentle movement which can help shift your body's balance from it's fight or flight response into a more relaxed state.
Calming of an overactive sympathetic nervous system requires, focusing on activating of your parasympathetic nervous system, which promotes rest and relaxation.
Breathing exercises that are good to stop an overactive sympathetic nervous system are deep belly breathing, physiological sigh and even using apps such as headspace or calm to help with guided breathing exercises.
Deep belly breathing is also called diaphragmatic breathing, which involves inhaling deeply and filling your belly with air first and then your chest and then exhaling slowly and reversing the process.
A physiological sigh is a quick way of re-calibrating your nervous system and involves 2 consecutive inhales through your nose, and then followed by a long exhale.
Practicing mindfulness and meditation can also help stop an overactive sympathetic nervous system.
Practicing mindfulness involves paying attention to the present moment without judgement, which can help to reduce anxiety and overthinking.
Meditation techniques such as focusing on a mantra or observing your breath, can also help calm your mind and body.
Other things that can help stop an overactive sympathetic nervous system are regular physical activity and gentle movement as well as cold exposure therapy, Tapping "EFT" and even use of weighted blankets.
Regular physical activity and exercise can help to regulate your nervous system and reduce system of stress and anxiety.
And doing activities such as tai chi, yoga and qigong can promote relaxation and also improve your nervous system regulation.
And brief exposure to cold like a cold shower or even a dip in cold water can stimulate your parasympathetic nervous system .
And emotional freedom techniques also known as EFT involves tapping on specific acupressure points, while also focusing on a specific issue, which can also help to release emotional blockages and calm your nervous system.