Do you live on the wrong side of your time zone? According to Dr. Sujay Kansagra, MD, a sleep specialist and child neurologist at Duke University, where you live within your time zone could be silently affecting your sleep, health, and even your income. A time zone is a region of the globe that observes a uniform standard of time for legal, commercial, and social purposes.
In a recent Instagram post, Dr. Kansagra highlighted findings from a fascinating research study that reveals significant differences in sleep patterns, health outcomes, and productivity depending on which side of a time zone you live in.
The concept is surprisingly simple yet profound. While everyone within the same time zone experiences the same clock time, the actual timing of natural light varies dramatically based on geography.
"Everybody in the eastern time zone is experiencing the same time," Dr. Kansagra explained in his post. "However, because the earth is round, people that live on the eastern part of the time zone are actually going to have sunset earlier than people in the western time."
To illustrate this point, he offered a concrete example: "If you live in Greenville, North Carolina, USA which is my hometown, sunset tonight is at 5:01 p.m. But if you live in Chattanooga, Tennessee, USA which is still in the eastern time zone, but it's the western part of the eastern time zone, sunset tonight is at 5:35 p.m."
That 34-minute difference in sunset time might seem trivial, but researchers discovered it has cascading effects on when people go to bed and how much they sleep.
The study Dr. Kansagra referenced examined bedtime patterns across the United States, and the results were striking.
Researchers found a clear pattern: as you move from east to west within a time zone, bedtimes consistently get later.
"This is so freaking cool," Dr. Kansagra said. "As you move across the time zone, as you go from east to west, bedtime tends to get later."
The consequences of living on the western edge of a time zone are more significant than you might expect. The research found that people living on the eastern part of a time zone sleep an average of 19 minutes more per night than those on the western edge of the same zone.
"And if you don't think 19 minutes a day is very much, it translates into 115 hours per year," Dr. Kansagra emphasized.
Those 19 fewer minutes of nightly sleep can be associated with serious health consequences, including higher risks of heart attack, obesity, and diabetes as the study noted.
Perhaps most surprisingly, the study also found a three percent decrease in wages among those getting less sleep.
Researchers used a spatial regression discontinuity design to examine what chronobiologists call "social jetlag" the mismatch between our biological clocks and social schedules.
This occurs because of the delayed onset of daylight and the biological link between environmental light and the production of melatonin throughout the day.
The term social jetlag is used to describe the discrepancy between biological time, determined by our internal body clock, and social times, mainly dictated by social obligations such as school or work.
This "forced synchronization" disrupts the natural release of hormones like melatonin, cortisol (“the stress hormone”), ghrelin (the “hunger hormone”), and leptin (the “satiety hormone”). These hormones regulate stress, metabolism, and inflammation, so their disruption can directly affect health by increasing risks of metabolic and cardiovascular diseases.
It also noted that individuals on the late sunset side of a time zone boundary will tend to go to bed at a later time.
Research also found evidence that people exposed to more evening sunlight tend to eat later and are more likely to dine out, contributing to obesity and diabetes risks.
The research findings extend beyond just sleep quantity. The study revealed significant effects on health outcomes typically associated with circadian rhythm disruptions that includes obesity, diabetes, cardiovascular diseases, and even breast cancer.
Researchers also observed impacts on economic performance, measured through per capita income.
The misalignment between biological and social time has real costs. Researchers calculated that circadian misalignment increases healthcare costs by at least $2 billion. Productivity losses from insufficient sleep induced by the extra evening light are equivalent to 4.4 million days of work.
1. Giuntella O, Mazzonna F. Sunset time and the economic effects of social jetlag: evidence from US time zone borders. J Health Econ. 2019 May;65:210-226. doi: 10.1016/j.jhealeco.2019.03.007. Epub 2019 Apr 13. PMID: 31030116.