U.S. researchers have found that employees who start work between 8 p.m. and midnight had consistently poorer performance at work, probably due to a lack of sleep.

The study found workers' sleep lasted from 4.5 hours to 8 hours, depending on when the shift began, said lead author Angela Bowen, research assistant at the Sleep and Performance Research Center at Washington State University Spokane.

It was predicted by the study that the minimum on-the-job fatigue occurred when the shift started at 9 a.m., while maximum fatigue occurred when the work shift began at 11 p.m.

Bowen said in a statement, "Shifts of equal duration differ in how fatiguing they are depending on the time of day when they are scheduled. 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."

It has been further reported that the study found those who had a shift begin at midnight did better because they could sleep right up to going to work, while those who started work earlier did worse because the timing conflicted with the body's early evening circadian process. (With Inputs from Agencies)