On-job fatigue highest when shift starts at 11 p.m.: study

Delaying the start time for an overnight shift could help late-night workers get more shut-eye off duty and feel less tired on the job, a new study suggests.

TORONTO — Delaying the start time for an overnight shift could help late-night workers get more shut-eye off duty and feel less tired on the job, a new study suggests.

A research abstract presented Tuesday at the annual meeting of Associated Professional Sleep Societies LLC in San Antonio, Texas, analyzed the effect shift start time had on workers’ predicted total sleep time off-hours, as well as on fatigue during a shift.

“Our findings were in two arenas: How much sleep they were able to get, and how fatigued they were during the shift,” said lead author Angela Bowen, a research assistant with the Sleep and Performance Research Center at Washington State University Spokane.

The study predicted the minimum fatigue at work took place during a shift starting at 9 a.m. The maximum on-the-job fatigue occurred with shifts starting at 11 p.m.

So why is it that someone clocking in at 11 p.m. is at a potential disadvantage compared to someone starting at midnight? Simply put, they’re not getting pre-shift sleep or enough of it.

Bowen said the theory is that everyone has a circadian rhythm, and that’s your body clock which is set with light exposure. That’s what makes it easier to either stay awake — or fall asleep — during different periods of the day.

“Most people can fall asleep really easily between about midnight and 6 a.m., that’s when your body is really ready to sleep for most people,” Bowen said.

On the flipside, the time period between 5 p.m. and 10 p.m. is sometimes called a “wake-maintenance zone” and is a period when it’s generally more difficult to fall asleep, she noted.

“If a shift starts at 11, the likelihood that you’d be able to take a good quality sleep before going on that shift is unlikely but at midnight, just that little hour later, it would be a little easier for you to get more sleep before that shift,” Bowen said.

The research analyzed was based on a two-process model of sleep regulation. That encompasses the circadian propensity for sleep and sleep-wake homeostasis. The principle behind homeostasis is that the longer you’re awake, the easier it is for you to fall asleep and the more you want and need sleep, Bowen said.

Researchers generated 24 set schedules where a person would be working nine-hour shifts, six days a week, and wouldn’t be able to sleep for the hour prior to or following their shift.

The study found with schedules starting between about 8 p.m. and midnight, the model predicted workers were getting about four-and-a-half hours of sleep and were very fatigued while on the job.

Those whose shifts started during daytime hours were catching far more zzz’s, according to predictions. Bowen said those with shifts starting between about 9 a.m. to 2 p.m. were getting about eight hours of sleep and weren’t as fatigued. Despite the predictions of less sleep time, it’s not all doom and gloom for graveyard shift workers. In fact, Bowen said one of the surprising findings was that for shifts starting at midnight or later, the model predicted workers would actually be better rested and less fatigued since they were able to sleep beforehand.

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