The window of circadian low occurs between which times?

• A. 0000 to 0400
• B. 0200 to 0559
• C. 0400 to 0800
• D. 0600 to 1000

Answer: (B)

The window of circadian low is when alertness and cognitive performance are at their lowest due to the body's internal clock, typically during the middle of the sleep period. This corresponds to the deepest dip in core body temperature and the peak in melatonin production, usually occurring in the early morning hours. In aviation medicine and related study material, this low period is defined as about 0200 to 0600 local time. The provided time span of 0200 to 0559 aligns with that standard, covering the core hours of lowest alertness. The other ranges either miss part of the deepest dip or extend into times when alertness has begun to rise, so they don’t reflect the true period of minimal performance.