“at lunch break” vs “in lunch break”

The sample sentence is actually quite ambiguously constructed. “Lunch break” is such a vague term, that makes it difficult for one to determine the “correct answer” in this scenario.

“At” is more frequently used in a spatial sense, though it also can be used in a time sense. It is used to convey a sense of precision. Since we are describing both location and time, “at” appears to be an acceptable option. However, the sentence feels “off”, probably due to the ambiguous message “at” sends. Is the author emphasizing on the timing or the location of the event (lunch break) to describe the action (read books)?

“In” is more related to a spatial description, but can also be used to describe time if applied towards a specific time period (e.g. This happened in the Cold War era). Given that “lunch break” is not a specific time period, and there is nothing that give hints to where the location of this event is actually within some enclosure, “in” is not a very proper way to construct this sentence.

Some suggested “on” as well in the comments, which is related to the phrase “on break”. However, “on” faces the same problem as “at” and “in” in that it can be used to describe both the time element and the space element of something. It also creates confusion with the way the sentence is constructed. (e.g. I read some books on the floor – is the message clear here? Grammatically, yes it refers to “I”, but it is not a very clear way to construct the sentence.) Though it is true that the sentence can only be logically interpreted as “on break”, “on” nonetheless is not a good choice here because of the ambiguity it creates. To use “on” to construct the sentence, you should clarify this ambiguity. (e.g. I read some books while I was on lunch break).

Some suggested “during”, which can only be used to describe time. Since ninjas do not exist in this world, “during” is actually the best option because it does not give ambiguous messages on whether the preposition is describing time or place. “I read some books during lunch break” automatically means that you are focusing on the time element of the lunch break, and it is understood that you are at wherever you are at during the lunch break when you say so. The clarity makes this the most preferable option of all.