First Sleep and Second Sleep

However, historical evidence, borne out by scientific studies, suggests that lying awake in the middle of the night was a normal part of the way our ancestors slept—so normal, in fact, that the night was broken into two chunks, “first sleep” and “second sleep.” The two sleeping periods were interrupted by an hour or two of quiet activity, in which people prayed, discussed their dreams, chatted about the day’s events, performed household chores, smoked, or had sex. Darker activities are also said to have been common during these late hours: jealous husbands alleged that their wives used the time to fly off to the witch’s Sabbaths; petty thieves took advantage of the darkness to steal from dockyards and orchards, among other crimes.

More here.

So perhaps Matins in the middle of the night for monks and nuns was not such an extraordinary thing to do.