You’ve likely heard of your body’s internal clock or circadian rhythm. But did you know there’s a physical location in the brain where this clock exists? It’s called the suprachiasmatic nucleus, or SCN for short, and it’s best thought of as your body’s master clock — the one that orchestrates the complex physiological processes involved in our daily cycles of sleep, waking, and activities. Synced to sunrise and sunset, it works to keep the body in balance, or homeostasis, explains Kenneth Wright, PhD, professor of integrative physiology and director of the sleep and chronobiology laboratory at the University of Colorado in Boulder.
But as it turns out, there are a lot of things we do that may be throwing off that balance.
Dr. Wright’s research investigates the health and safety consequences of poor sleep and disrupted body clocks. His work has contributed to a growing field of evidence that time-shifted schedules, overly long workdays, intermittent or oddly timed meals, and reliance on artificial light long after sunset have increased our risk for disease, chronic health problems — like diabetes, obesity, and heart disease — dysfunction, and deterioration.
“Circadian misalignment, not living according to the natural clock, interferes with biological pathways involved in immune function, metabolism, cancer, and with altered glucose and energy metabolism,” Wright says.
“In women, we see evidence of this in the effects of overnight shift work, which increases the incidence of breast and endometrial cancers.”
In a phone interview, Wright explained more about our circadian rhythms and the role they play in getting a good night’s sleep and overall, long-term health.
Everyday Health: What is our body clock and what does it do?
Kenneth Wright: Our circadian rhythm, which most of us refer to as the body clock, is actually controlled in the suprachiasmatic nucleus [SCN] located in the brain’s hypothalamus — and synced to solar day and night. It predicts and prepares for our daily activities, coordinating the bodily functions that accompany those activities from the time we wake up through our sleep.
This master controller orchestrates the autonomic nervous system, which controls bodily functions we’re not conscious of (like breathing and heartbeat). It also coordinates the release of cortisol, the hormone thought to prepare the body for the waking day, and it directs the release of melatonin, the hormone that signals to the body it’s time to sleep about two hours before our normal bedtime.
The human SCN’s roughly 100,000 neurons coordinate the many cellular biological clocks that exist throughout the body, ensuring that the right sequence of events occurs for every activity [each day]. The presence of these clocks — which exist in our fat cells, liver, pancreas, muscles, and organs — was not common scientific knowledge until the early 2000s.
EH: So, we actually have multiple internal body clocks — but the master clock controls them all? How are all of these clocks connected?
KW: We’ve learned that while there is a master clock in the brain, almost every cell in the body has its own biological clock, too. Each cell keeps time through the expression of certain genes and proteins [becoming active] at certain times during a 24-hour period. The explanation of that mechanism earned a Nobel Prize in 2017 and, since then, other researchers have gone on to find these clocks everywhere across the body.
In the natural world, sunlight is the strongest time cue for these clocks. It sets the timing of the master clock, which then coordinates the processes that occur throughout the body, including the early morning processes that promote wakefulness, as well as when we eat, when we are active, and the processes that control sleep.
Another example: The master clock prepares the body to eat by ensuring that our cells and tissues work together in anticipation of meal time. It also varies body temperature during the day. It’s purpose [that of the master clock] is to keep things operating in synchrony.
EH: In addition to all the functions that occur while we’re awake, our master body clock also controls when we sleep, right?
KW: Yes, the circadian rhythm promotes sleep at night in humans. In the natural world, our physiology is matched to daylight, so that when light diminishes, the processes that prepare us for sleep are triggered. During sleep itself, the SCN remains involved in the peak timing of the different stages of sleep (including REM, the rapid eye movement sleep), and at dawn it initiates the changes that increase alertness.
Light has always been critical to this circadian process, but it’s historically been natural light from the sun.
As we’ve removed ourselves from direct exposure from sunlight during the day, spending time indoors at work or at school, we’ve separated ourselves from receiving important bodily cues.
When we come home at night and turn on bright lights, we again interfere with the natural cycle. Bright lights during the evening and night tend to push the biological clocks later than normal.
EH: What do you mean by “push the clock later”?
KW: It means that we’re not getting the circadian signals to our body to prepare for sleep. The cues are timed later, and so we’re having more trouble falling asleep and, subsequently, waking in the morning for work and school. The research conducted by my team [published in April 2013 in the journal Proceedings of the National Academy of Sciences (PNAS)] has shown that this circadian misalignment leads to lower insulin sensitivity — one reason why short sleep may elevate risk of diabetes.
Short sleep duration and misalignment of natural circadian rhythm are also believed to contribute to obesity, metabolic syndrome, heart disease, mood disorders, cognitive impairment, and sleepiness-related accidents.
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EH: Does the master clock differ for men versus women?
KW: In the broadest possible terms, men and women differ in the speed of their biological clocks. (I’m referring to biological sex in answering this, rather than self-reported gender.) A faster clock leads to an earlier timed circadian rhythm.
We know from research that the SCN’s control of melatonin and temperature, both of which affect sleep, run over a shorter day-night period in young adult women than in men. That means that women — women in the aggregate, that is — have a circadian cycle that is roughly 30 minutes shorter relative to their sleep time than men [causing the cycle to start that much earlier in the day].
EH: You’re saying that women’s body clocks run, on average, 30 minutes ahead of schedule everyday compared with men’s clocks? How does this difference affect our functioning?
KW: This “running ahead of schedule” makes women more likely to be "morning people." As a result, they are better matched to the typical business and school day, where you tend to wake up earlier and go to bed earlier. Conversely, men tend to do better later and have less trouble with overnight shift work than women. It can also impact how people feel throughout the day.
We know from our [2013 PNAS] research that when women and men get the recommended amount of sleep (at least seven hours) and are allowed to eat as much as they want, men overeat and gain weight, whereas women tend to eat what they need and maintain weight. However, when men and women both do not get enough sleep, dietary restraint is reduced for both sexes.
We think that eating more during too-short sleep is a biological adaptation that occurs as the body seeks the energy it needs to maintain wakefulness.
We also know that the interconnected functions of the body are no longer properly coordinated when the circadian rhythm is disrupted. For instance, eating at night alters the timing of the biological clock cells in the liver, so that it is out of sync with the master clock, as well as overall physiology. We have physiological processes that are meant to work with others, and those mechanisms are thwarted. As a result, over time, eating at biologically inappropriate times taxes the body and can lead to some serious health problems.
EH: Do women and men have different sleep patterns (when we’re actually asleep) because of the differences in our body clocks?
KW: We know that children, both boys and girls, get the most deep sleep around age 12, when we have the highest number of brain cells. This heightened sleep is required to prune these brain cells during the growth period.
After puberty, girls and boys start to go to bed later because it’s easier for them to stay awake longer, with their lengthening circadian cycle. At this time, research reveals that boys adopt later bedtimes than girls — becoming later still in the early twenties.
As adults, statistics [from the U.S. Department of Health and Human Services] show that 1 in 4 women develop insomnia symptoms, which makes them 10 times likelier to later report depression and 17 times more susceptible to anxiety issues. Meanwhile, it’s believed that men account for more sleep apnea cases until women are postmenopausal.
This sex difference in sleep timing is maintained throughout adulthood until around age 50, when men and women converge a bit more in their sleep, as well as in their sleep problems. By age 60, research suggests an estimated 57 percent of the general population reports a sleep complaint.
It’s also worth noting that beyond this general rule that the female clock is timed earlier, men and women don’t have different body clocks. Their circadian rhythms are more similar than different.
EH: Are there other differences (besides those determined by circadian rhythm) that distinguish the way men and women sleep?
KW: Although their circadian rhythms are pretty much the same, there are differences in the way men and women sleep. These begin in infancy, when baby boys face a higher risk for sudden infant death syndrome that infant girls.
As they enter puberty, girls can have their sleep interrupted by severe premenstrual syndrome and its painful cramps. Polycystic ovarian syndrome (PCOS) in females is another sex-related problem of hormonal imbalance, and it tends to be associated with an increased risk of sleep apnea.
And, again, insomnia is more prevalent among women, where it also increases the risk for depression. Sleep-related eating disorder (SRED), a condition where people involuntarily eat and drink during the nighttime sleep period, tends to affect more women than men. There’s some speculation that it could be related to hunger caused by daytime dieting.
EH: We’ve covered a lot, including the point that a lot of our daily routines could be interfering with how our body clock naturally functions. Do you have any tips for keeping the clocks running on time?
KW: I’d like people to stop shortchanging sleep. I’m often asked whether it’s more important to get an extra half-hour of sleep or spend 30 extra minutes working out. I think they’re both important. And if we have to take time away from something, let it be the TV, computer, or the smartphone screen. Sleep and exercise are both critical to maintaining a healthy lifestyle.
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Remember, there was a time not too long ago when we couldn’t convince people that smoking was unhealthy. The general public came around slowly, but now everyone accepts that it causes disease. I think we’ll see the same thing happen with sleep.
Those who understand that shortchanging sleep is unhealthy but haven’t yet figured out a way to establish good sleep habits themselves will eventually take time from other activities and devote it to sleep. And for those who still struggle, we have good treatments that can fix sleep problems and promote sleep and circadian health.
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Until then, I’d like to emphasize daylight and its impact on circadian system. We need bright light in the morning and soft, diminishing light as we approach bedtime. We should do our best to get out and soak up sunshine early. And if it’s not possible, at least exercise near a window and turn up the lights during the daytime, while taking care to reduce lighting as we get closer to bedtime. Remove electronic devices from the bedroom and stop using them within an hour or two of sleep.
Remember; health is a three-legged stool. You have to eat well, be active, and get adequate sleep and daylight to ensure good health. Eliminate one of the legs and the stool breaks.