Under What Specific Conditions Is It Rational to Exercise an American Call Option Early?

It is generally only rational to exercise an American Call option early if the underlying stock is about to pay a large dividend. Exercising before the ex-dividend date allows the holder to capture the dividend payment.

However, the loss of the option's remaining time value must be less than the dividend amount for the early exercise to be economically beneficial. On non-dividend-paying stocks, early exercise is never optimal.