What's the best work schedule for your age?

The moment you may have been dreading is here. Lazy summer days have come to an end, and more structured back-to-work or back-to-school schedules have commenced.

But will your new daily schedule be the healthiest for your brain and body?

Scientists have long known that you have an internal biological clock that regulates various physical, mental and behavioral changes your body experiences over a 24-hour cycle, called circadian rhythms. Those circadian rhythms can slowly change as you age.

So you tend to require different work schedules and work hours throughout your lifetime. From your adolescence to post-40, here are the schedules that research suggests may be the best for your age:


Teens and young adults: Later may be better

Even though you are legally an adult when you turn 18, your brain may not grow out of adolescence until your mid- to late 20s, said Dr. Jess Shatkin, a psychiatrist at the Child Study Center at NYU Langone Medical Center.

As a result, "adolescents have this desire to go to bed later and wake up later, and that's what most people do until they hit around 26," Shatkin said, and it partly comes down to melatonin, a hormone linked to your biological clock that influences when you start to naturally feel sleepy at the end of the day.

"Adolescents start to release melatonin later in the day than adults. They release around 10 o'clock at night, naturally, which means they then get sleepy later, and because they go to bed later, they wake up later," Shatkin said. He added that the amount of melatonin we release might drop by about 50% once we hit puberty.

So then, what would be the ideal work or school schedule for adolescents? "It's hard to say what's ideal, because people vary, of course," Shatkin said, "but I would say that for adolescents we really shouldn't be starting school (or work) before 9 a.m. and, ideally, start at 10 a.m. ... and let the schedules go later."

In a 2014 policy statement, the American Academy of Pediatrics recommended that middle and high schools delay the start of classes to 8:30 a.m. or later to align with the biological sleep rhythms of adolescents.


Mid-20s and 30s: Shifting away from shift work

For most adults in their 30s and 40s, some studies suggest that ideal work schedules should mirror individual preferences, whether you are an early riser or a night owl. Research also indicates that these preferences are linked to your genes.

But whether you're more of a morning or evening person, many studies consistently show that irregular or overnight shift work can have a negative impact on your health.

"It is important to check on your familial predisposition for diseases that have been previously linked to working irregular shifts," said Christian Benedict, a researcher in Uppsala University's department of neuroscience in Sweden.

"In other words, do your grandparents or parents have type 2 diabetes, obesity or heart diseases? If yes, it might be better to go for a '9-to-5' schedule," he added. "If you are forced to working irregular shifts for the sake of money, make sure that you are physically and mentally active."

After all, shift work can impair not only your body but also your brain, according to a new study that Benedict co-authored and published in the September issue of the journal Neurobiology of Aging.

For the study, self-reported data from more than 7,000 adults in Sweden were analyzed. The data then were compared with how each adult performed on a two-part test designed to screen for cognitive impairment, called the trail making test.

The researchers discovered that current shift workers and those who worked irregular shifts in the past five years were more likely to have a poorer performance on the test than their counterparts.

"In contrast, no difference was observed between non-shift workers and those who had quit shift work more than five years ago," Benedict said.

"The latter could suggest that it may take at least five years for previous shift workers to recover brain functions that are relevant to the performance on this test," he said. "However, as of yet, a mechanistic explanation as to why shift workers require at least five years of time-out to recover their cognitive performance is lacking, and as such warrants further investigation."

Sufficient sleep benefits the brain since, as you snooze, the organ can flush out cellular waste that builds up during wakefulness, previous research has shown.

Since shift workers often experience sleep disturbances or may not get enough sleep, their brains might not effectively remove waste, Benedict said.

"Some of these waste products can be harmful for your neurons," Benedict said.


40 and older: The 25-hour work week

Once adults are older than 40, however, the number of work hours packed into a daily schedule may have health effects similar to the schedule itself.

Research published in the Melbourne Institute Working Paper Series in February suggests that a three-day workweek could be best for adults 40 and older -- but that's the sweet spot.

Working more or less than about 25 hours each week could have negative impacts on cognitive functioning, said Shinya Kajitani, associate professor at Meisei University in Japan and a co-author of the paper.

Just last year, some companies in Sweden trimmed 40-hour work weeks to 30 hours for full-time employees, and they reported that workers were less fatigued, more efficient and happier.

"Work can stimulate brain activity, but longer working hours are more likely to cause physical and/or mental stress," Kajitani said.

"The point we are making in our paper is that work can stimulate brain activity and can help maintaining cognitive functions for elderly workers," he added. "At the same time, excessively long working hours can cause fatigue and physical and/or psychological stress, which potentially damage cognitive functioning."

The researchers analyzed household, income and labor data on about 3,000 men and 3,500 women 40 and older. They then compared that data with how the men and women performed on cognitive tests.

The researchers found that working up to 25 hours a week was linked to an improvement in cognitive performance, but when working hours exceeded 25 hours a week, there was a decrease in cognitive performance.

"We are a little bit surprised because our findings -- that the net positive effect of work peak around 25 hours a week and start declining thereafter -- are well captured for non-linearity in the effect of working hours on cognitive abilities," Kajitani said.

Could such working hours be linked to similar results in other age groups?

Women in their 20s, 30s and 40s who work an average of 60 hours or more per week may triple their risks of diabetes, cancer, heart trouble and arthritis, according to a separate study published in the Journal of Occupational and Environmental Medicine last month.

Additionally, a 2015 study published in the Lancet found that working more than 55 hours per week may be linked to an increased risk of heart disease and stroke in adults.

But to really examine differences in age groups, Kajitani said, more research is needed.

"We speculate that the positive impact of working hours on cognitive functioning may be different among the age groups, but unfortunately, we have not produced any evidence for this yet," Kajitani said. "We are currently doing some work on this issue."