What was the man’s excuse for being tired?

The reason the man stated for being tired was that he had a late night. This suggests that his fatigue likely stemmed from staying up later than usual, possibly due to activities that kept him awake, like socializing or working on personal projects. This reason is directly related to his rest and sleep schedule, implying that his lack of sleep resulted in his tiredness.

The other options, while they could also explain tiredness, do not directly indicate that the tiredness was specifically due to the lateness of the previous night alone. Working late, having an early meeting, or exercising may contribute to fatigue but are not as clearly related to the notion of a "late night" as an excuse. In this case, the specific phrasing of "late night" makes it the most appropriate choice for this context.