Glorious, refreshing sleep is eluding the majority of Americans. According to the National Sleep Foundation’s 2013 International Bedroom Poll 56 percent of people between the ages of 25 and 55 get an insufficient amount of sleep on workdays. On non-workdays individuals are then more likely to oversleep. They spend an additional 45 minutes catching Z’s in an attempt to compensate for accrued workweek sleep debt. Why are we constantly playing sleep-catch up during free time?
As a society we are socially jet lagged. Social jet lag is the difference betweensleep patterns on work days and free days. These inconsistent sleeping habits result in sleep loss that is reminiscent of flying west across several time zones every Friday evening and traveling back East come Monday morning. The pattern reveals a critical disparity between society-imposed obligations, like work and family commitments, and our innate biological clock. Social jet lag might not sound like a big deal. What’s an hour or two of sleep lost here and there? But the chronic misalignment between our social and biological clocks is wreaking havoc on our health.
Large-scale epidemiological studies have pointed a finger at short sleep duration for it’s causative role in the nationwide obesity crisis. When you get too little sleep, normal levels of appetite hormones are altered in a way that could lead to increased food consumption and weight gain. Unfortunately for people struggling with social jet lag, short sleep duration comes with the territory of the workweek. Some data even suggest that for every hour the biological clock is offset from the social clock, the chances of being overweight shoot-up by a whopping 33 percent. And supersizing the body mass index isn’t the only problem. Social jet lag has also been linked to the increased likelihood of nicotine and alcohol use, which independently contribute to additional health problems.
The social jet lag epidemic tends to hit teenagers particularly hard. Their biology hardwires a preference for late night schedules that clash with early school start times. A recent study published in the Journal of Developmental & Behavioral Pediatrics suggests that starting school a mere 25 minutes later can significantly improve adolescents’ mood and sleep duration while reducing caffeine use and daytime sleepiness. Many states have already begun championing delayed high school start times and some districts have reported declines in teen driver crash rates following new schedule implementation.
If the number of hours spent snoozing each day is critical for a healthy lifestyle, is society cheating more than half of Americans out of essential sleep with an unsustainable social schedule? For the more than half of Americans experiencing social jet lag, the answer is a resounding yes. But what about the other 44 percent? Why do some people thrive off of early morning schedules or late night shift work while others flounder? Whether you’re a morning lark, a night owl, or something in between, it’s a matter determined by a complex milieu of biological and social factors that involve subtle differences in biological clock genes.
Dual systems collaborate behind forty-winks
Throughout the day, two complimentary processes are at work regulating the balance between alertness and sleepiness: the sleep drive and the circadian rhythm. Sleep drive acts as a monitoring system that tells our bodies when sleepiness is building-up. Once a critical amount of sleepiness has accumulated, it informs us that it’s time for bed and helps us stay asleep through the whole night. If sleep drive were the only process impacting sleep-wake timing, we would feel most awake in the morning and get increasingly sleepy as the day went on. Luckily, there is a second process that keeps us from falling asleep face-first into our lunchtime salad bowl – the circadian clock.
Ever been hit by the afternoon slump? Time drags, the coffee line at Starbucks stretches out the door, and your head begins the traditional dance of Bob. That drowsy, fuzzyheaded feeling felt by many people around 3:00 PM is a dip in the natural circadian rhythm. This internal biological clock regulates the timing periods of alertness and sleepiness experienced throughout the day.
Run by a master clock made up of 10,000 neurons located in the anterior portion of the brain’s hypothalamus, the suprachiasmatic nucleus (SCN) manages the circadian rhythm. The SCN is influenced directly by environmental cues, such as sunlight, that signal a 24-hour cycle. When it receives light/dark information from the eye, the SCN signals other parts of the brain to regulate how sleepy or alert you feel. For example, in the morning when you wake up and first greet the sun, the SCN sends out a message to raise your body temperature and suppress the release of melatonin, a hormone that is associated with the onset of sleep.
Why we sleep when we sleep: A genetic link?
It’s more than just a personal choice; both sleep drive and the circadian clock influence your diurnal preference to be morning person or an evening person.
In 1976, researchers Horne and Östberg were the first to show that subtle differences in body temperature and food intake throughout the day were predictive of a person’s preference for morning or evening. Morning-types had an earlier rise and steeper peak in temperature compared to evening types, despite both groups getting the same amount of sleep. And in a weird twist, subsequent studies showed that although morning-types wake up at an earlier time of day, they wake later relative to the circadian cycle of temperature and melatonin. Evening-types exhibit the reverse scenario.
Diurnal preference also manifests as differences in normal sleep. Electroencephalography sleep studies that measure electrical activity on the scalp as a correlate of neuronal activity in the brain have found that morning-types show differences in slow-wave sleep activity over the first two NREM/REM sleep cycles. They also spend more time in stage 1 sleep than evening-types.
What’s behind all of these differences? Identical twin studies have shown that morning-evening preference has a heritability of around 50 percent. And while the exact mechanisms have yet to be fully parsed out, along with contributions from external and social factors, polymorphisms – variation in the genetic code – of circadian clock genes have been linked to diurnal preference.
A number of genes have surfaced as potential candidates for influencing a person’s morning-evening preference. To date, the most studied is a polymorphism in the clock gene PERIOD3 (PER3). In humans, PER3 contains a segment of 54-nucleotides that repeats four (PER34) or five (PER35) times. Different diurnal preference characteristics are observable depending on how many repeats you have in your gene.
Studies have found that individuals homozygous for the PER35 allele (PER35/5) – meaning they have two copies of the same longer polymorphism – wake up earlier and prefer mornings. They also go to bed earlier and spend less time in bed on workdays. On non-workdays, those with PER35/5 sleep longer, suggesting they might be particularly susceptible to the trend of social jet lag if they don’t get to bed early enough during the week. Notably, one study found that the PER35/5 individuals had a higher body mass index (BMI) if the midpoint of their sleep was later than 4:15 AM on workdays. However, if it was before 4:10 AM on free-days BMI was significantly lower. Understanding correlations, such as the dynamic relationship between genotype and BMI, may be key in developing interventions for sleep issues like social jet lag.
Garfield the cat famously quipped, “If people were meant to pop out of bed, we’d all sleep in toasters.” This comic strip character’s lack of enthusiasm for mornings highlights a reality for many people with social jetlag: springing out of bed bright-eyed and bushy-tailed just isn’t in the cards. While we might not be able alter our genetics to best fit our social schedule, some experts think society should focus on changing policies to align work and school schedules better with biological time. Until sleep is acknowledged as a critical factor impacting the country’s overall wellbeing, one strategy some individuals with social jet lag may benefit from is getting outside in the morning and soaking in the sun (with the correct SPF on, of course). It might help some people fall asleep earlier and get more hours of sleep. Or we could all just figure out how to fit our pillows into the toaster’s slots.
Roenneberg T., Allebrandt K., Merrow M. & Vetter C. (2012). Social Jetlag and Obesity, Current Biology, 22 (10) 939-943. DOI: 10.1016/j.cub.2012.03.038
Boergers J., Gable C.J. & Owens J.A. (2014). Later School Start Time Is Associated with Improved Sleep and Daytime Functioning in Adolescents, Journal of Developmental & Behavioral Pediatrics, 35(1) 11-17. DOI: 10.1097/DBP.0000000000000018
Lázár A.S., Slak A., Lo J.C.Y., Santhi N., von Schantz M., Archer S.N., Groeger J.A. & Dijk D.J. (2012). Sleep, Diurnal Preference, Health, and Psychological Well-being: A Prospective Single-Allelic-Variation Study, Chronobiology International, 29 (2) 131-146. DOI: 10.3109/07420528.2011.641193
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