Answer:
In this study carried out by Kelly and colleagues, and published in 1999, called: "Nonentrained Circadian Rythms of Melatonin in Submariners Scheduled to an 18-Hour Day", the results showed that the human body tends to always adjust its circadian rythm to 24-hour cycles, sometimes a bit longer, and they respond normally to different stimuli, like lights, clocks and social interaction. However, on these submariners, from the Trident, a nuclear submarine, who had been forced to adjust to 18-hour schedules, the production of melatonin, the sleep-inducing, and circadian-cycle mediator, and the mediation of the weak-sleep process, would still follow a 24.35 hour cycle. This, of course, produces problems with the circadian rythm and it also showed that these submariners´ circadian rythms would not respond to external stimuli, like clocks and social contact, when forced to adapt to thte 18-hour cycle.
Explanation:
Humans are geared to respond to the cycles of light and dark, and the production of melatonin, to either wake up, or sleep. This wake-sleep process follows a pattern and a cycle, which normally follows a 24-hour rule. When schedules mess with this cycle, through the use of bright artifical lights, and other stimuli, this process is affected, producing a lot of stress-like reactions from the body itself. In these submariners that were studied, it was seen that the circadian cycle would still follow the 24-hour pattern, but the officers were forced to adapt to an 18-hour cycle, which meant that negative effects were brought to their bodies, as they were not working under natural conditions.
