GROTON, Conn. -- With no sunlight to set day apart from night on a submarine, the U.S. Navy for decades has staggered sailors' working hours on schedules with little resemblance to life above the ocean's surface.

Research by a Navy laboratory in Groton is now leading to changes for the undersea fleet. Military scientists concluded submarine sailors, who traditionally begin a new workday every 18 hours, show less fatigue on a 24-hour schedule, and the Navy has endorsed the findings for any skippers who want to make the switch.

The first submarine to try the new schedule on a full deployment was the USS Scranton, led by Cmdr. Seth Burton, a cancer survivor. He said the illness he experienced as a junior officer helped convince him of the health benefits of keeping a sleep pattern in line with the body's natural rhythm.

"I know that there's lots of medical side effects to just not having a good, regular sleep pattern," said Burton, 41, of Huntsville, Ala.

A submarine sailor's day is generally divided equally into three periods: Time on watch, off time that is devoted partly to training and drills, and sleep. Under the new schedule, those time blocks are stretched from six to eight hours.

Submarine crews are not big enough to support more than three watch rotations and, beginning in the 1960s, the Navy capped shifts at six hours in part to limit fatigue as sailors manned the vessels' nuclear reactors. But the study by the Naval Submarine Medical Research Laboratory, at the submarine base in Groton, documented weariness that can set in every third cycle as sailors are working when their bodies are accustomed to sleeping.

Navy Capt. Steven Wechsler, the laboratory's commanding officer, said the study found the fatigue that came from working on the reactors an additional two hours can be balanced out by the longer, more consistent sleep period on the 24-hour schedule.

Since 2005, the laboratory has done experiments on submariners' sleep patterns, testing melatonin levels in sailors' saliva, surveying crews and fitting sailors with devices to measure activity levels and sleep quality. Last May, the Navy authorized submarine commanders to use the 24-hour schedule. Wechsler said he expects submarines will use it "when appropriate," noting it may depend on the mission type.

The circadian rhythm, a master biological clock that regulates when we become sleepy and when we're alert, has been the subject of many studies by industry and academia. The Navy's surface fleet is also trying schedules that align more with the natural body clock: A strike group deployed with the aircraft carrier George H.W. Bush is trying a schedule of three hours on, nine hours off.

The Groton lab focused specifically on applications for submariners, one of the only groups outside a laboratory to operate without any external time cues.

While the medical benefits may be clear, the transition to a 24-hour schedule poses logistical challenges on cramped submarines.

On the attack submarine Scranton, which returned in January to Norfolk, Va., from a seven-month deployment, Burton said the new schedule initially led to backlogs of laundry and frustrations over access to laptops and exercise equipment. The enlisted sailors on Burton's crew kept a straight, eight-hour rotation, but he structured shifts for officers in a way that allowed all of them to be awake and work together for part of each day.

He said sailors always managed to adapt to the old schedule, but after working out the wrinkles, the new hours were well received.

"The crew loved it," he said. "I saw a great response."

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