> they generally live a ... life without hard labor

Monks work, a lot, both in order to maintain the monastery itself and to produce goods for the outside community.

You can easily see this for yourself in Eastern Europe: except for a few highly touristic sites, you are generally welcome to stay at a monastery for a time without being asked to pay anything, but you will be expected to assist the monks in the work they do each day. The work – from chopping wood to weeding gardens – is exhausting. It is a mystery to me how the monks manage to keep up with everything and maintain a very positive attitude, all on 4 hours of sleep a night and diet that is fasting half the year.

Physical labour, especially when it feels valuable and productive, is incredibly fulfilling despite the physical exhaustion that comes with it.

Unlike mental exhaustion, physical exhaustion makes it very easy to fall asleep and sleep deeply. This is the real secret to the monks' success. Those of us who tax our minds with knowledge work don't get the benefits of physical exertion that the monks enjoy.

> all on 4 hours of sleep a night and diet that is fasting half the year.

not sure where you got 4 hours of sleep. Monks generally wake up early but they also retire early.

For example: http://www.stmarysmonastery.org/daily_life.html wake at 4:35, retire (though not necessarily sleep) at 20:15. The schedule has about 6~7h of work tops:

* following Mass + Terce + community meeting of 0930 (so probably about 1000~1015) until 1300

* following the Office of Nones (which starts at 1500) until 1800, and that can also be studies

The schedule you are describing is a Roman Catholic one. I am speaking about Orthodox monasteries. Often monks will be expected to attend one last hour of prayer around midnight. Following that, they retire, but then they will have to return to the church very early the next morning for matins, and apparently they wake up a couple of hours before that for personal devotions. As a guest at monasteries, I have been allowed slightly less than 6 hours of sleep because I am expected to attend all hours of prayer, so if the monks are waking up even earlier, then 4 hours for them sounds about right.

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.

Monasteries of the medieval / renaissance time period usually had very large complements of lay staff and tenant farmers, who maintained the estates and produced goods for both internal use and sale at market.

This is not to say the monks didn't work or didn't follow ascetic regimens, but they weren't doing all the work of keeping the monastery running.