**Core Concept**
In atrial fibrillation, the heart's normal pacemaker (sinoatrial node) is overridden by chaotic electrical activity in the atria, resulting in an irregularly irregular heart rhythm (typically 100-160 beats per minute). However, in this question, the patient has a regular beat of 60/min, which suggests an underlying mechanism that is stabilizing the heart rate.

**Why the Correct Answer is Right**
Digitalis toxicity is a well-known cause of a regularized atrial fibrillation rhythm, often referred to as "digitalis-induced junctional tachycardia." This occurs when the toxic effects of digitalis (a heart medication) cause the atrioventricular (AV) node to become overactive, taking over as the heart's pacemaker and producing a regular heart rate. The AV node is normally capable of pacing the heart at a rate of 40-60 beats per minute, which is why digitalis toxicity can sometimes result in a regular beat in a patient with atrial fibrillation.

**Why Each Wrong Option is Incorrect**
**Option A:** Sleep is not a known cause of regularized atrial fibrillation rhythm.
**Option C:** Sino nodal block would typically cause a heart rate drop, not an increase, and is not associated with regularized atrial fibrillation.
**Option D:** Hypothyroidism can cause bradycardia, but it is not typically associated with regularized atrial fibrillation rhythm.

**Clinical Pearl / High-Yield Fact**
In patients with atrial fibrillation, always consider digitalis toxicity as a potential cause of a regularized heart rate, especially if the patient is on digitalis medication and presents with a regular heart rate.

**✓ Correct Answer: B. Digitalis toxicity**