Athlete's heart: This condition is common in young, healthy individuals who are physically active. The heart adapts to the increased demand by slowing the heart rate and increasing stroke volume, which can result in a heart rate in the 30s during rest.

Vasovagal syncope: Although this condition typically presents with a sudden loss of consciousness, it can also cause bradycardia. The mechanism involves a reflex of the involuntary nervous system, causing blood vessels to dilate and heart rate to slow.
Hypothyroidism: This condition can cause a decrease in metabolic rate, leading to a decrease in heart rate. It is more common in women and can present with a variety of symptoms, including fatigue, weight gain, and cold intolerance.
Sleep: Bradycardia can be a normal finding during deep sleep, especially in young, healthy individuals.

Myocardial infarction: Although rare in young women, myocardial infarction can cause bradycardia, especially if there is involvement of the atrioventricular node.
Cardiac conduction abnormalities: Conditions such as complete heart block or sick sinus syndrome can cause significant bradycardia and may require urgent medical attention.
Hypothermia: Accidental or environmental hypothermia can cause bradycardia and is a life-threatening condition if not promptly recognized and treated.

Infiltrative cardiac diseases: Conditions such as amyloidosis or sarcoidosis can infiltrate the heart and cause conduction abnormalities, including bradycardia.
Neurological disorders: Certain neurological conditions, such as increased intracranial pressure or brainstem lesions, can affect the autonomic nervous system and cause bradycardia.
Medication side effects: Certain medications, such as beta-blockers or calcium channel blockers, can cause bradycardia as a side effect.