Wondering if biphasic sleep was involved with those schedules -- but it is not obvious to us now? https://en.wikipedia.org/wiki/Biphasic_and_polyphasic_sleep#... "Historian A. Roger Ekirch has argued that before the Industrial Revolution, interrupted sleep was dominant in Western civilization. He draws evidence from documents from the ancient, medieval, and modern world. Other historians, such as Craig Koslofsky, have endorsed Ekirch's analysis. According to Ekirch's argument, adults typically slept in two distinct phases, bridged by an intervening period of wakefulness of approximately one hour. This time was used to pray and reflect, and to interpret dreams, which were more vivid at that hour than upon waking in the morning. This was also a favorite time for scholars and poets to write uninterrupted, whereas still others visited neighbors, engaged in sexual activity, or committed petty crime. The human circadian rhythm regulates the human sleep-wake cycle of wakefulness during the day and sleep at night. Ekirch suggests that it is due to the modern use of electric lighting that most modern humans do not practice interrupted sleep, which is a concern for some writers. Superimposed on this basic rhythm is a secondary one of light sleep in the early afternoon."

A point to consider with regard to the abbreviated sleep schedule of monks is the restive and restorative aspects of the meditative periods during each day. For Catholic and Orthodox monks the periods of centering, contemplative, and some other types of prayer are the same as or comparable to other meditation practices which have documented restorative value. From my experience the more time spent in meditation the less sleep needed. Even doing hard labor, digging ditches and building stone walls, during a meditation retreat provide enjoyable engagement that support the still times. Anecdotally, placement of prayer just before sleep facilitates a more rapid entry into both sleep and deep sleep.

Many of them (if not all) simply nap during those hours of meditations and prayers.

Again, that might be true of Roman Catholic monasteries. However, Orthodox worship is more “active”. First of all, worshipers stand during services, there are no pews except a handful of chairs for the elderly or infirm. Secondly, monks are expected to sing the sung portions of the liturgy (there is no choir at a monastery, they are the choir) and to perform all of the many genuflections involved. I have occasionally seen 1–2 monks in their dotage being permitted to sit and doze during prayers, but everyone else is expected to remain awake and do everything that needs to be done.

I have friends in Eastern Orthodox monasteries. They nap a lot during prayers.