106. Read the following passage, then answer the question below.

Nearly every living thing, from bread mold to humans, follows a daily rhythm of activity that continues even without clues from the outside world. In 1962, the French cave explorer Michel Siffre tested this idea by living for two months in a cave in the Alps, with no clock and no daylight. He slept and woke by his own sense of time, and he found that his body settled into a cycle of roughly a day, though not exactly twenty-four hours. Later experiments with larger groups confirmed that the internal period is close to, but usually slightly longer than, a solar day. This is why the rhythms are called circadian, from Latin words meaning 'about a day.'

If the internal clock runs a little long, something must reset it daily, and that something is light. Specialized cells in the retina detect brightness, especially the blue-rich light of morning, and send signals to a cluster of neurons in the hypothalamus that acts as a master clock. This cluster coordinates the timing of hormones, body temperature, and alertness throughout the body. In the evening, as darkness approaches, the brain releases melatonin, a hormone that prepares the body for sleep.

Problems arise when daily schedules clash with the clock. Travelers crossing time zones experience jet lag until their bodies shift to the new light pattern, and the adjustment is often estimated at about a day for each hour of difference. Shift workers face a more persistent mismatch, and studies link long-term night work to higher risks of heart disease and diabetes. Even exposure to bright screens late in the evening may delay melatonin release, although researchers disagree about how large the effect is. What is clear is that the clock does not respond to willpower. It follows light, and ignoring it carries a cost.

The word 'it' in paragraph 2 refers to