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If a shift starts at 11 p.m., it is less likely that someone will get more high-quality sleep and be rested for work, researchers say. ((iStock))

An overnight shift that starts after midnight may help workers feel less tired, a study suggests.

A research summary to be presented Tuesday at the annual meeting of the Associated Professional Sleep Societies in San Antonio, Texas, predicts minimum on-the-job fatigue for different shifts, based on a mathematical model of sleep time outside of work.

The maximum estimated sleep duration, eight hours, occurred when the work shift started between 9 a.m. and 2 p.m. The minimum estimated sleep duration, 4.5 hours, occurred when the shift began between 8 p.m. and midnight, the researchers said.

There was also a relatively sudden decrease in predicted fatigue for work schedules starting after midnight compared with shifts that had workers clock in just before midnight.

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That's because work schedules with start times after midnight allowed workers to get more high-quality sleep before going to work, said study lead author Angela Bowen, a research assistant with the Sleep and Performance Research Center at Washington State University Spokane.

But if a shift starts at 11 p.m., it is less likely that someone would get good quality sleep and be better rested for work.

Shifts that started just before midnight did not allow for pre-shift sleep because that timing conflicts with the body's early evening circadian rhythm, which is set with light exposure.

The circadian rhythm makes it easier for most people to fall asleep between midnight and 6 a.m., while between 5 p.m. and 10 p.m., the body is generally trying to maintain wakefulness, Bowen said.

"Shifts of equal duration differ in how fatiguing they are depending on the time of day when they are scheduled," Bowen said in a release. "The same limitation on the number of duty hours may be either overly restrictive if during the day or too liberal if during the night."

Based on the findings, the researchers suggested employers schedule shift start times in a way that maximizes sleep and alertness.

In particular, night shifts that begin after midnight may allow workers to get more sleep and experience less fatigue than shifts that start between 8 p.m. and midnight, they said.