Is sinus bradycardia always a sign of an underlying health issue?

Absolutely not!: Virtually all seasoned athletes have sinus bradycardia. Many normal, healthy non-athletes do as well. Virtually everybody has SB at sometime during sleep.

Usually not: As long as sinus bradycardia doesn't cause symptoms like dizziness, then it's not a problem. Well-trained athletes can have impressive sinus bradycardia because their hearts in such good shape, they don't need to beat as fast as the rest of ours. Rarely sinus bradycardia can be caused by critical illness, but in those cases the severity of illness is obvious.