'Nocturnal job shifts cause most fatigue'

The research, based on a mathematical model of sleep time outside of work, however, found that an overnight shift that starts after midnight may help workers feel less tired. The maximum estimated sleep duration occurred when the work shift started between 9 am and 2 pm.

According to lead author Angela Bowen of the Sleep and Performance Research Centre at Washington State University Spokane, the work schedules that start after midnight allow workers to get more high-quality sleep before going to work.

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.

For the study, the team generated 24 hypothetical work schedules with shift start times that varied by one-hour increments across the 24-hour clock. Each hypothetical work schedule lasted six days and allowed for no sleep during duty periods.