Genetic clocks
Chronotypes and Social Jetlag
Image credit Alexis Webb
When I talk to people about my research field, I tell them that I want to understand how organisms tell time. Our cells are equipped with genetic clocks that help coordinate our temperature, metabolism, and hormones when we are awake and asleep. In fact, most things on the planet keep daily time, and have evolved very similar ways of doing so that persist even without changes in our environment. These near 24-hour patterns in behavior and physiology are called circadian rhythms. In mammals, a pair of nuclei comprised of about 20,000 neurons is the tissue responsible for coordinating time for the rest of the body. Sitting in the hypothalamus at the base of the brain is a pair of nuclei that act like a conductor of the symphony of instruments that comprises our suite of biological processes.
Once this conversation about rhythms starts, the next question often is: can you tell me why I’m having problems x, y, z with my sleep? It’s hard to ignore the impact that sleep has on our lives, given that we spend about a third of our time doing it. Sleep is controlled by two drives: our need for sleep and our circadian rhythm.
Humans are diurnal animals; we are awake during the day and asleep at night. Our internal intrinsic clock sets us to that pattern by receiving input from the environment. But modern society has thrown in factors affecting our lives that evolution hasn’t necessarily prepared us for. Electricity, work schedules, the constant entertainment of television and the Internet – all of these interfere with our internal ability to keep time. This is a problem that has recently been coined “social jetlag” and is tied to higher incidences of mood disorders and obesity. Till Roenneberg, professor of chronobiology at the Institute of Medical Psychology at Ludwig-Maximilians University in Munich, Germany, has been at the forefront of studying circadian rhythms in human populations discusses social jetlag in his book, Internal Time: Chronotypes, Social Jetlag, and Why You’re So Tired as well as a YouTube video his daughter produced.
Image credit Alexis Webb
What kind of effect does social jetlag have on the average person? Roenneberg surveryed thousands of individuals on their sleep and wake preferences using the Munich ChronoType Questionnaire (MCTQ). He and others have shown that most adults get about one hour less sleep than they would when they don’t have an alarm. At the extreme end of the distribution, “night owls” or those scored as late chronotypes, find the difference between sleep duration on work and free days is nearly 2.5 hours (note: this number is not corrected for any sleep debt that is accrued during the week). That so-called “lie in” on the weekends may simply be a more natural time to wake up for most people. A vastly different relationship between internal clock time (circadian) and external social time (work schedule) could be why some find it hard to get up during the week.
The relationship between internal and external time is defined as the phase of entrainment. Chronotype allows researchers to describe this relationship in a quantitative way using a particular phase marker, mid-sleep on free days. The distribution of chronotypes centers on a mean phase of entrainment, but the spread of extremes spans more than half the day. This method takes into consideration the social constraints that are imposed on our circadian rhythms and is a good metric to describe how we organize our daily behavior. There are differences in chronotypes across the population by age, as well as by location. For example, adolescents tend to delay their chronotype as they get older, peaking on average by around 20 years old, before starting to shift earlier again. It’s also true that people over 60 have earlier chronotypes than most children.
Our internal clock runs autonomously but requires an external signal like light or temperature to entrain to local time. We rely on these signals to be zeitgebers (German for “time giver”). By programing an alarm to wake you, you are ignoring the phase of entrainment given by the strongest zeitgeber we know: sunlight. What happens to sleep and entrainment when these modern stimuli are removed? Researchers led by Kenneth Wright, Associate Professor in the Department of Integrative Physiology at the University of Colorado, studied how natural light acts as a signal to our circadian clocks by monitoring behavior of 8 individuals during a weeklong camping trip in the Rocky Mountains. When day-to-day activity takes place in electric lighting plus additional natural light, sleep time started at around 12:30 a.m. and wake time occurred around 8:00 a.m. After a week of exposure to natural light only, all markers of internal circadian time occurred ~2 hours earlier. This included the levels of the hormone melatonin, which peaked during the middle of the night with onset at dusk and offset at dawn. This was a significant difference in melatonin cycle under electric lighting conditions, where onset is shifted to approximately 2 hours before sleep instead of at dusk. These studies provide strong evidence that modern living does play a role in altering normal sleep and circadian behavior.
My goal in writing this blog is to communicate topics in chronobiology with readers. I hope to approach these topics in informal, but informative ways, giving readers a glimpse at cutting edge research in the context of modern life. Through images, video, and audio, I will try to describe science through a creative and playful lens. I hope to add elements of personal timing into the mix, as an early career researcher trying to find my path. Follow me on Twitter @lexbwebb.