Holy basil (Ocimum sanctum or Ocimum tenuiflorum), also known as tulsi, has been used for centuries in Ayurvedic medicine as an adaptogen—a substance believed to help the body cope with stress and support homeostasis. Traditional Ayurvedic texts describe holy basil as a herb that calms the mind and body, supporting a balanced nervous system, which is consistent with the modern concept of promoting parasympathetic (rest-and-digest) activity. The leaves and extracts have been used to reduce symptoms of stress, anxiety, and fatigue, which are often associated with overactivity of the sympathetic nervous system and underactivity of the parasympathetic branch.

While animal studies and a limited number of human trials suggest that holy basil may reduce stress markers (such as cortisol) and promote subjective feelings of well-being, robust scientific evidence directly linking holy basil to enhanced parasympathetic nervous system function is lacking. Most of the current support comes from traditional usage and small-scale studies rather than large, high-quality clinical trials. Nonetheless, the traditional rationale aligns with the herb’s adaptogenic and anxiolytic properties as observed in preliminary research, which may indirectly support parasympathetic activity by mitigating stress responses.

In summary, holy basil's use for supporting the parasympathetic nervous system is primarily grounded in traditional medicine, with moderate, but not definitive, supporting evidence from modern scientific research.