Sinus bradycardia is seen in:
Sinus tachycardia is associated with a sinus rate of more than 100/min. The various causes are,

Anxiety
Fever
Exercise
Emotion
Anaemia
Thyrotoxicosis
Beta adrenoceptor agonists

Sinus bradycardia is associated with a sinus rate of less than 60/min and is seen commonly in athletes.

Myocardial infarction
Hypothyroidism
Cholestatic jaundice
Raised intracranial pressure
Beta-blockers
Digoxin
Verapamil
Athletes
Exercise
Thyrotoxicosis
Beta adrenoceptor agonist