Window of circadian low
Window of circadian low means a period of maximum sleepiness that occurs between 0200 and 0559 during a physiological night.
14 CFR § 117.3
In addition to the definitions in 1.1 and 110.2 of this chapter, the following definitions apply to this part. In the event there is a conflict in definitions, the definitions in this part control for purposes of the flight and duty limitations and rest requirements of this part.