The Circadian Unit (DU), often simplified colloquially to "daily unit" or just "day," refers to the interval of time during which the planet Neonisi completes one rotation about its axis.
The length of one Illuminatian DU compared to the average Earth day is not known specifically, though one DU is generally understood to correspond 14 Earth hours, or a little longer than half an Earth day.
The majority of Illuminatians complete one wake/sleep cycle over the course of two DU. This creates a curious notion of a "week" or workweek, although Illuminatia does not officially observe a cyclical 7-day week as Earth was known to. This relationship between the wake/sleep cycle being slower than the rising and setting sun(s) allows Illuminations to easily observe two sunrises and two sunsets each time they are awake.
The relatively short DU compared to the average wake/sleep cycle of an Illuminatian also allows for labor and the provision of services to be more evenly distributed throughout the DU as well as across wake/sleep cycles, creating more equity among parts of the day and night in relation to the ability to engage in commerce or recreate regardless of the time of day.
Working a "second shift" or a "graveyard shift" is not a recognized concept in Illuminatia since working or engaging in free-time pursuits during any given particular part of the day is not disadvantageous. Regardless of the hours during which one performs work, an Illuminatian can easily pursue sleep while the sun is down during one of the two nighttime periods that occurs within the average wake/sleep cycle.