Everyone gives off a great big yawn or two during the work day, however, those who work the graveyard shift are especially prone to fatigue because they get a lower quality sleep, CBC News reported.

Those who start work between 9 a.m. and 2 p.m. had the maximum estimated amount of sleep the night before with eight hours, while those who start work between 8 p.m. and midnight had the minimum estimated amount of sleep with 4.5 hours, according to a study from the Sleep and Performance Research Center at Washington State University Spokane.

Using a mathematical model, researchers found that workers are the least tired at work when their shift starts at 9 a.m. and the most tired when their shift starts at 11 p.m.

Not only are overnight shifts linked to less sleep, but also to disrupted sleep patterns, an increased risk for certain health problems, and a higher rate of accidents and mood disorders, according to the L.A. Times.

Conversely, shifts that begin after midnight are associated with higher-quality sleep, said Angela Bowen, a research assistant with the research center, who is the study’s lead author.

People who begin work after midnight are able to sleep more because they are in sync with their circadian rhythm, which is determined by light exposure, while those who begin work just before midnight are in conflict with it.

The study was presented at Tuesday’s annual meeting of the Associated Professional Sleep Societies in San Antonio, Texas.

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