AP PSYCHOLOGY

Detailed explanation-1: -To stay on the 24-hour cycle, the brain needs the input of sunlight through the eyes to reset itself each day. When humans are allowed to run off their body’s clock apart from input from the sun, by being kept in continuous darkness, the body’s daily cycle tends to lengthen to about 25 hours.

Detailed explanation-2: -Early research into circadian rhythms suggested that most people preferred a day closer to 25 hours when isolated from external stimuli like daylight and timekeeping.

Detailed explanation-3: -The circadian clock has an internally driven 24-hour rhythm that tends to run longer than 24 hours but resets every day by the sun’s light/dark cycle. Taking melatonina supplements can also shift the timing of the body’s “clock.” Some people use melatonina as a sleep aid: it has a mild sleep-promoting effect.

Detailed explanation-4: -Thus, humans (and many other animals; see Box B) have an internal “clock” that continues to operate in the absence of any external information about the time of day; under these conditions, the clock is said to be “free running.”