According to a study by Kelly et al. (1999), naval personnel on submarines were asked to function on 18-hour "shifts": 6 hours of work, 6 hours of recreation, and 6 hours of sleep. What happened to their circadian rhythms?

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.