American self-improvement writing gives the first hours of the day disproportionate attention, and describes them in language of discipline and character. The emphasis has identifiable sources.
Religious orders scheduled the day around dawn
Monastic rules across several traditions assign fixed prayers to the hours before and around sunrise, and rising in darkness is treated as part of the discipline itself.
Because these communities also organized labor, study and meals around that schedule, the early hours acquired an association with seriousness of purpose.
That association traveled into general culture through institutions shaped by religious orders, including schools, hospitals and universities.
Agrarian and industrial schedules reinforced it
Farm work is bound to daylight, and rising early was a practical necessity rather than a virtue in agricultural communities across the country.
Industrial shift work then fixed early starts by clock rather than by sun, and lateness became a documented offense with wages attached.
By the time the schedule was no longer necessary for most work, the moral coloring had already attached itself to the hour.
Attitudes toward idleness supplied the vocabulary
Strands of American Protestant thought treated diligence in one's ordinary work as a religious matter, and idleness as a spiritual danger rather than a preference.
Sleeping late fits that framework as visible evidence of the wrong disposition, which is why the criticism is so often about character rather than about rest.
The vocabulary persisted after the theology receded, so contemporary advice uses moral language while offering productivity reasons.
The practical case is narrower than the moral one
The defensible argument is about interruption: early hours are quiet because other people are asleep, and uninterrupted time is scarce for most adults.
That argument is about the absence of demands rather than about the hour, and it applies equally to late-night hours for people whose households allow it.
Research on circadian variation indicates that natural timing differs substantially between individuals and shifts with age, which the moral framing ignores entirely.
Why contemplative traditions still specify a time
Most traditions that prescribe daily practice fix an hour, and the reason given is usually consistency rather than any property of the hour itself.
A practice attached to a specific time survives competition with other obligations, while one scheduled whenever convenient tends to lose that competition.
On that reasoning the early hour is chosen because it is defensible against interruption, not because virtue is distributed unevenly across the day.
Teachers who work with shift workers and parents of infants usually say so directly, assigning a fixed time that fits the household rather than importing a schedule designed for a monastery.