Cold water on your face in the morning
Splashing cold water on your face, or finishing your shower with 30 seconds of cold, activates the diving reflex, a hardwired autonomic response that immediately slows heart rate and boosts vagal tone. A 2018 study published in Frontiers in Physiology found that cold water face immersion produced a measurable increase in HRV within seconds by stimulating the vagus nerve through trigeminal nerve endings. You don't need a cryotherapy chamber. The kitchen tap works.
