American Journal of Epidemiology Advance Access originally published online on March 3, 2008
American Journal of Epidemiology 2008 167(6):755-756; doi:10.1093/aje/kwn002
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LETTERS TO THE EDITOR |
THE FIRST AUTHOR REPLIES
Royal Alexandra Hospital for Children, Westmead, New South Wales 2145, Australia
(e-mail: lawrencl{at}chw.edu.au)
I am thankful for the interest and comments by Li and Zhang (1) on our article (2). Li and Zhang have raised a few interesting points in their letter that I would like to discuss further.
The observation regarding the stress experienced by high school students in China, particularly relating to the highly selective education system, is vivid. This phenomenon is common in some countries with similarly highly competitive education systems, including Australia. It is totally understandable that high school students in these countries are under stress and may experience various physical and/or psychological problems, including those related to sleep.
Although sleep problems may be associated with chronic stress, it is a rather reductionist view to consider that the association between short duration of sleep and unintentional injuries simply reflects a relation between chronic stress and injuries, as asserted by Li and Zhang (1). The article by Voelker (3), quoted by Li and Zhang to support their argument, stated that students' increased sleep problems and mounting reports of depression on college campuses are intimately connected (3). An epidemiologic interpretation of such a statement is that there is a significant association between sleep problems and depression among college students without invoking the concept of causality or the direction of causality. This interpretation does not convey any directional meaning such as "sleep deprivation can be rooted in stress." Moreover, the sleep problems of college students highlighted in Voelker's article, as referenced to Pallesen et al.'s (4) and Hicks et al.'s (5) reports, included insomnia, use of hypnotics, sleep onset problems, and general dissatisfaction with sleep. These descriptions of sleep pathologies differ from shortening the duration of sleep by choice or because of other nonmedical reasons.
As mentioned in the Discussion section of our article (2), accumulated sleep debt has impacted significantly on human physiologic rhythms such that the body manifests a depression-like physiologic response (6). The underlying physiologic mechanism is believed to be related to rapid eye movement sleep (REMS). It has been established that REMS is associated with depression in humans, where a deprivation of REMS has an antidepressive effect and that most antidepressant medications suppress REMS (7). This relation between REMS and depression is possibly attributable to a neuroanatomic and neurochemical process (8). Sleep disruption or deprivation might interrupt REMS and shorten the duration of REMS during normal sleep. However, as the human body is deprived of REMS, it compensates with prolonged REMS, which is known as REMS rebound. Frequent sleep disruptions during normal sleep when a person is asleep or deprivation of sleep may effect more frequent REMS rebound, hence increasing the risk of depression.
In terms of the relation between stress and sleep pattern, a recent study demonstrated that psychological stress significantly enhances the total amount of REMS by prolonging the average duration of REM sleep in the animal model (9). Hence, one can theorize about the associations among stress, sleep problems, and depressive mood, where an increase in psychological stress is linked to an increase in REMS and in turn an increase in depressive mood. However, according to the Diagnostic and Statistical Manual of Mental Disorders: DSM-IV (10), one of the principal diagnostic criteria for a major depressive episode is that the person experiences either insomnia or hypersomnia. In other words, a depressive person may find it difficult to sleep or sleeps too much. This is again different from shortening sleep duration by choice or because of other nonmedical reasons. Hence, shortening one's sleep is not merely a reflection or manifestation of stress. To my knowledge, so far there is no substantial evidence in the literature to demonstrate the strong association between chronic stress and short duration of sleep, as suggested by Li and Zhang (1).
In terms of the role of age or school grade, as suggested by Li and Zhang (1), the relation between short duration of sleep and unintentional injuries is rather irrelevant, as indicated by our results (2). We found no associations between age and the outcome or the exposure (tables 2 and 3). According to the criteria for a confounding factor discussed by Rothman and Greenland (11), age or its surrogate does not satisfy the requirements. Therefore, it is unlikely that age or its proxy would exert any effect on the relation between short duration of sleep and injuries.
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ACKNOWLEDGMENTS |
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Conflict of interest: none declared.
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References |
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 TOP References |
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- Li Y, Zhang J. Re: "Short duration of sleep and unintentional injuries among adolescents in China." (Letter). Am J Epidemiol (2008) 167:754–5.
[Free Full Text] - Lam LT, Yang L. Short duration of sleep and unintentional injuries among adolescents in China. Am J Epidemiol (2007) 166:1053–8.
[Abstract/Free Full Text] - Voelker R. Stress, sleep loss, and substance abuse create potent recipe for college depression. JAMA (2004) 291:2177–9.
[Free Full Text] - Pallesen S, Nordhus IH, Nielsen GH, et al. Prevalence of insomnia in the adult Norwegian population. Sleep (2001) 24:771–9.[Web of Science][Medline]
- Hicks RA, Fernandez C, Pellegrini RJ. Striking changes in the sleep satisfaction of university students over the last two decades. Percept Mot Skills (2001) 93:660.[CrossRef][Web of Science][Medline]
- Spiegel K, Leproult R, Van Cauter E. Impact of sleep debt on physiological rhythms. Rev Neurol (Paris) (2003) 159(suppl):6S11–20.[Medline]
- Thase ME. Depression and sleep: pathophysiology and treatment. Dialogues Clin Neurosci (2006) 8:217–26.[Medline]
- Germain A, Buysse DJ, Wood A, et al. Functional neuroanatomical correlates of eye movements during rapid eye movement sleep in depressed patients. Psychiatry Res (2004) 130:259–68.[CrossRef][Web of Science][Medline]
- Cui R, Suemaru K, Li B, et al. The effects of atropine on changes in the sleep patterns induced by psychological stress in rats. Eur J Pharmacol. Advance Access: October 6, 2007. (1796 1548).
- American Psychiatric Association. Diagnostic and statistical manual of mental disorders: DSM-IV. (1994) 4th ed. Washington, DC: American Psychiatric Association.
- Rothman KJ, Greenland S, eds. Modern epidemiology. (1998) 2nd ed. Philadelphia, PA: Lippincott Williams & Wilkins.
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