"Shortage" or "shortening"?

SAD(seasonal affective disorder) is a response to the shortening of daylight hours and lack of sunlight in winter.

The sentence above is extracted from the book that I'm studying by myself right now named Grammar and Vocabulary for Advanced by Martin Hewings and Simon Haines. The answer is shortening,however, I cannot understand as I think we can use shortage in this case.

Can you please help me? Thanks

Solution 1:

Actually, I agree with you. Shortage would make more sense, although it would imply something different from what the author meant.

Shortening refers to something getting shorter, in this case it is the day (as in "the part of the day during which we receive sunlight", opposed to night) that gets shorter. The daylight hours, however, do not get shorter. They are still 60 minutes long.

The number of hours during which we receive sunlight, however, decreases, thus causing a shortage of such hours.

Purely grammatically, within our current system of defining hours as being of a constant length, I agree that shortage makes more sense than shortening.

But...

If we read daylight hours as a descriptive form of "day" (the light part, as opposed to night), it does get shorter, and shortening of days makes grammatical sense.

Furthermore, using shortage would imply a different situation: if I experience a shortage of daylight hours, I would assume, for some reason, that I'm not getting enough daylight because, for instance, I'm staying inside too much. I would not associate that with the changing of the seasons. Lengthening and shortening of the days I do associate with the changing seasons.

Therefore, I understand the author's choice for shortening, even though the days get shorter, and not the hours.

On a side note, in ancient Rome, this sentence would have made more sense (provided it were written in Latin...). In Rome, the day was divided into twelve hours between sunrise and sunset, meaning that an hours did not always have the same length. Under that system, one could indeed observe that in fall, the hours would shorten.