Olivia Judson - A New York Times Blog
April 21, 2009, 10:00 pm
Guest Column: Larks, Owls and Hummingbirds
By Leon Kreitzman
Teenagers are notoriously difficult to rouse in the mornings. For the sake of parental authority it may be best that we keep this an adult secret, but . . . it may not be the youngsters’ fault.
In many cases, it is not laziness, but a part of normal development and determined by the genes. Human circadian clocks are often geared to “owl-like” behavior during adolescence. Fortunately, boys tend to grow out of this by about age 20 and girls a year earlier. But there is a good case for schools opening later and experimenting with the timing of the curriculum and of examinations, so that there is a better match between organizational requirements and the capabilities of the students.
Lark by Daniel Pettersson, Creative Commons (some rights reserved); Spotted Owl by Don Ryan/The New York Times; hummingbird by Yuri Cortez/The New York Times Which one are you?
The teenagers are doing what teenagers do because, left to our natural devices, we would eat, sleep and drink (along with many more biological functions) not when we decide to, but when our biological clock tells us to. Only cultural norms and the alarm clock give us the pretense of choice by overriding our inner rhythms — and there is increasing evidence that we are paying a high cost in terms of our health. Disruption of the circadian clock is linked with cancer, cardiovascular diseases, gastric illnesses, asthma, schizophrenia, learning disorders and other conditions.
Charles Czeisler of Harvard University established a decade ago that the mean for the intrinsic, or “free-running,” activity rhythm is 24 hours 11 minutes, plus or minus 16 minutes. It is not 24 hours 11 minutes for everyone, of course, as there is considerable variation among individuals and over the course of one’s life.
Czeisler’s arduous study involved a month’s intensive monitoring of 29 adults. The participants were denied time cues (food, for instance, was made available more or less constantly), kept in a semi-recumbent posture and in a “forced” 28-hour sleep-wake cycle. This complex and carefully controlled regime was necessary to disassociate the intrinsic rhythm from the normal light signals that synchronize the rhythm to the solar cycle.
Our circadian rhythm is synchronized with the 24-hour solar cycle by light signals received by non-rod, non-cone receptors found exclusively in our eyes. While the 70-80 percent of us who are dubbed hummingbirds manage this well, the “larks,” who tend to be up early, and late-rising “owls,” who turn in usually well after midnight, have difficulty resetting their internal clocks.
Larks tend to be older; college students and twenty-somethings are well-known owls. Larks are most aware around noon, work best in the late morning and are chatty, friendly and pleasant from about 9:00 a.m to around 4:00 p.m. Owls, on the other hand, do not really get going until the afternoon, are at their most pleasant (if that is not an oxymoronic term for college students) later in the day and are at their most alert after 6:00 p.m.
It might be envy on my part, but those early-rising larks I have known have often seemed to my bleary early-morning eye to adopt a smug moral superiority based on Benjamin Franklin’s maxim, “Early to bed and early to rise makes a man healthy, wealthy, and wise.” But there is no basis for Franklin’s claim. Catharine Gale and Christopher Martyn of Southampton University followed up a 1973 survey that had included data on sleeping habits. More than 20 years later they found no evidence among the survivors that following Franklin’s advice was associated with any health, socioeconomic or cognitive advantage.
If anything, owls were wealthier than larks, though there was no difference in their health or wisdom. Gale and Martyn wryly offer the thought that “it seems that owls need not worry that their way of life carries adverse consequences. However, those who cite Franklin’s maxim to encourage their children to go to bed early may wish to consider whether their practice is entirely ethical.”
This evening/morningness is less a matter of choice than of genetics. Being bright-eyed and raring to go first thing in the morning is not just a case of how much sleep someone has had, nor is it a reflection of willpower. Genes may largely determine it.
While at the University of Utah, Louis Ptácek and colleagues studied three families with familial advanced sleep-phase syndrome (FASPS), an extreme form of lark behavior. One family included a grandmother, daughter and grandchild, all with the same sleep disturbance. The family members with the disorder have an aberrant wake/sleep cycle. Regardless of work schedules or social pressures, they cannot stay up much later than 7:30 p.m. and they tend to wake up around 3:30 a.m. In other words, the disorder shifts the normal wake and sleep pattern forward by three to four hours.
By studying the family relationships, Ptácek found that the disorder is inherited, and he has found the genes involved. The genetic implications prompted two senior researchers to comment, “It seems that our parents — through their DNA — continue to influence our bedtimes.”
Apart from its importance in helping to understand the relationship between the circadian clock and the sleep process, the work on FASPS was the first time that scientists have uncovered a genetic mutation leading to a change in a complex human behavior like sleep.
The traditional classification of people into larks, owls and hummingbirds may be too simplistic. A study of a large sample of the workforce at a Volkswagen car plant suggests that people fall into a spectrum of chronotypes between the extremes, depending on a range of factors, notably their genetic makeup and the amount of light they are exposed to during the day. This last may be much less than many people think. In brightly lit offices, the light levels are some 200-300 times less than they are outside on a sunny day. Even a cloudy day is some 20-30 times brighter.
Bright light has a powerful effect in shifting the phase of our body clock, and if we don’t see much bright light — and many office workers do not — then our circadian health suffers. One idea is that commuter buses and trains should have glass roofs, so that at least some of the workforce will get a daily dose of outdoor light.
Till Roenneberg, a professor at Ludwig Maximilians University in Munich, has developed a robust questionnaire that determines an individual’s proclivity to “morningness” and “eveningness.” He has coined the phrase “social jet lag” to describe the persistent mismatch between people’s biological clocks and the demands of their jobs or education.
It is bad enough for the larks and owls whose genes cause a mismatch between modern life and the ancient human body clock. For hummingbirds working night shifts or burning the proverbial candle at both ends, the implications are far-ranging concerning learning, memory, vigilance, performance and quality of life.
Society pays far too little attention to circadian disorders. Roenneberg’s call to employers to say to their workforce, “Please wake up in your own time and come in when you are ready,” is provocative but a challenge that we have not been facing up to.
Humans have broken many links with the natural world. Our food comes pre-packed, our drink pre-bottled and we take pills instead of chewing leaves. Electricity turns our nights into days, and central heating our winters into spring. But if we go deep into a dark cave without a watch, after a few days we revert to ancient patterns. Deprived of time cues, our rhythms slowly drift out of alignment with the outside world.
Over 20 percent of the working population now work at least some of the time outside the normal 7:00 a.m. to 7:00 p.m. working day, and the trend is increasing. Living outside the “natural” circadian pattern has undoubted health risks.
But we have choices. We can use what we know about the molecular mechanisms of circadian rhythms to mitigate the biological harm of our “24/7” world. We could better manage the effects of this world by re-organizing work patterns and schedules. We could match chronotype to suitable schedules. We could try to create a world in which we can offer a time paradise or “Uchronia” for a time-stressed populace.
Or is it too late, and we have become trapped in the materialistic time hell of “Dyschronia”?
For more on determining the human circadian rhythm as 24 hours 11 minutes plus or minus 16 minutes see Charles A. Czeisler, Jeanne F. Duffy, Theresa L. Shanahan, Emery N. Brown, Jude F. Mitchell, David W. Rimmer, Joseph M. Ronda, Edward J. Silva, James S. Allan, Jonathan S. Emens, Derk-Jan Dijk, Richard E. Kronauer. “Stability, Precision, and Near 24-Hour Period of the Human Circadian Pacemaker,” Science, June 25, 1999, Volume 284, pp. 2177-2181.
On the link between multiple sclerosis and vitamin D see Ramagopalan SV, Maugeri NJ, Handunnetthi L, Lincoln MR, Orton S-M, et al. (2009) Expression of the Multiple Sclerosis-Associated MHC Class II Allele HLA-DRB1*1501 Is Regulated by Vitamin D. PLoS Genet 5(2): e1000369.
Michael Smolensky and Lynne Lamberg applied the term hummingbird in “The Body Clock Guide to Better Health,” Owl Books 2000
Benjamin Franklin’s motto was tested in Gale, C. & Martyn, C. (1998) “Larks and owls and health, wealth, and wisdom.” Br Med J, 317, 1675–77.
Our parents determining our bedtime, see Singer, C. M. & Lewy, A. J. 1999 “Does our DNA determine when we sleep?” Nat Med, 5, 983.
A discussion of the importance of chronotypes is T. Roenneberg, A. Wirz-Justice, M. Merrow “Life between Clocks: Daily Temporal Patterns of Human Chronotypes Journal of Biological Rhythms,” Vol. 18, No. 1, 80-90 (2003)
A description of the molecular basis of larks and owls is at Steven A. Brown, Dieter Kunz, Amelie Dumas, Pål O. Westermark, Katja Vanselow, Amely Tilmann-Wahnschaffe, Hanspeter Herzel, and Achim Kramer “Molecular insights into human daily behavior” Proc Natl Acad Sci U S A. 2008 February 5; 105(5): 1602–1607.
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