People whose work shifts start between 8 pm and midnight are most likely to suffer from on-the-job fatigue as they get the least time to sleep, a new study claims, advocating for flexibility in job timings that could maximise the workers' alertness.
The research, based on a mathematical model of sleep time outside of work, found that those who report to duty between 8 pm and midnight get only 4.5 hours of sleep, which causes fatigue and affects their alertness.
However, it found that an overnight shift that starts after midnight may help workers feel less tired.
The findings showed that estimated sleep durations varied from 4.5 hours to eight hours according to the start time of the work shift.
The maximum estimated sleep duration occurred when the work shift started between 9 am and 2 pm, and the minimum estimated sleep duration occurred when the shift began between 8 pm and midnight.
"Our most interesting finding was that shifts beginning between 8 pm and midnight yielded consistently poorer predicted performance and less than adequate predicted total sleep per 24 hours," said lead author Angela Bowen of the Sleep and Performance Research Centre at Washington State University, Spokane.
But, there was a relatively decrease in expected fatigue found in people whose job shifts started after midnight.
According to Bowen, the work schedules that start after midnight allow workers to get more high-quality sleep before going to work.
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. Such timings don't allow for pre-shift sleep because that timing conflicts with the body's early evening circadian rhythm, which is set with light exposure, Bowen said.
The circadian rhythm, she said, makes it easier for most people to fall asleep between midnight and 6 am, while between 5 pm and 10 pm, the body is generally trying to maintain wakefulness.
"Shifts of equal duration differ in how fatiguing they are depending on the time of day when they are scheduled," she 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."