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Regular Article   |    
Lack of Seasonal Mood Change in the Icelandic Population: Results of a Cross-Sectional Study
Andres Magnusson, M.D.; Johann Axelsson, D.Phil.; Mikael M. Karlsson, Ph.D.; Högni Oskarsson, M.D.
Am J Psychiatry 2000;157:234-238. doi:10.1176/appi.ajp.157.2.234

Abstract

OBJECTIVE: The prevalence of seasonal affective disorder—as measured by the Seasonal Pattern Assessment Questionnaire—has been found to be unexpectedly low among Icelanders. The aim of this cross-sectional study was to measure seasonal variations in the prevalence of anxiety and depression among Icelanders assessed with the Hospital Anxiety and Depression Questionnaire. METHOD: Four 1,000-person cohorts, age 20–70 years, selected at random from the Icelandic National Register, were sent the Hospital Anxiety and Depression Scale by mail in either January, April, July, or October. Only responses from the 4-week period after each mailing were considered in the subsequent analysis. RESULTS: The mean anxiety and depression scores in winter were not higher than those in summer for either sex. There was no significant difference between winter and summer in rates of actual or borderline cases of anxiety or depression or for the two categories combined. CONCLUSIONS: This lack of seasonality in anxiety and depression is in sharp contrast to findings from similar cross-sectional studies and may reflect the low propensity for seasonal affective disorder that has been described in the Icelandic population.

Abstract Teaser
Figures in this Article

The epidemiology of seasonal affective disorder has been intensively studied in recent years, with three major approaches having been used. One approach has been to examine patient populations by searching case registers for individuals with a clear seasonal pattern in their relapses and remissions. A second approach has been to screen populations for current symptoms at different parts of the year, then examine seasonal changes. A third approach has been to survey populations with an instrument that focuses directly upon seasonal changes in mood and behavior. The Seasonal Pattern Assessment Questionnaire (1) is such an instrument and has been used to identify subjects with seasonal affective disorder as well as its milder form, subsyndromal seasonal affective disorder (2, 3).

Most studies have found seasonal affective disorder and subsyndromal seasonal affective disorder to be more common in women than in men and that the incidence of both conditions decreases with age. Several studies have also found that the prevalence of seasonal affective disorder increases with latitude (48). However, some studies have suggested that people may acclimatize to the long winters experienced at higher latitudes (7, 9). The question that needs to be addressed is whether the propensity for seasonal affective disorder differs among genetically diverse groups. Seasonal affective disorder and subsyndromal seasonal affective disorder have been studied in Iceland (latitude 64°N –67°N), where, unexpectedly, they were found to be less common than at much lower latitudes along the East Coast of the United States (39°N –42.5°N) (10, 11). When compared to the rate reported in the Montgomery County study in the United States (3), the prevalence of seasonal affective disorder has been found to be lower than expected in Japan (5, 12). Furthermore, in contrast to other populations, seasonal affective disorder is less prevalent among women and the young within the Japanese population (1214). Genetic factors have been suggested as an explanation for these differences (13).

To test the hypothesis of a significant, etiological genetic component, seasonal affective disorder was studied in a population of immigrants of wholly Icelandic descent in Canada (10). The prevalence of seasonal affective disorder and subsyndromal seasonal affective disorder was significantly lower in this immigrant population than in the aforementioned East Coast population. We have recently shown a striking difference in the prevalence of seasonal affective disorder between descendants of Icelanders in Winnipeg and other citizens of Winnipeg (15). All of these studies used the Seasonal Pattern Assessment Questionnaire. The advantage of this tool is that it focuses directly upon seasonal variations in mood and behavior. The disadvantage is that it relies upon the participants’ long-term recall of their seasonal mood swings. The reliability of recall of affective episodes may be poor (16), and the recall of seasonal pattern of depressive episodes may also be poor (17).

The aim of the present study was to examine seasonal variations in mood in the Icelandic population by methods that do not depend on the long-term recall of the participants. For this purpose, we used the Hospital Anxiety and Depression Scale (18) to measure the prevalence of anxiety and depression during summer, autumn, winter, and spring in a representative sample of the Icelandic population. The results are compared to cross-sectional studies from other countries.

Four cohorts of 1,000 persons each, aged 20–70 years, were selected at random from the Icelandic National Register. After obtaining necessary ethical approval, each cohort was mailed the Hospital Anxiety and Depression Scale in either January, April, July, or October of 1987. A cover letter and a self-addressed stamped envelope were enclosed. The participants were asked to rate their state of well-being in the week before receiving the questionnaire. The dates were specified. Two follow-up letters were mailed. Only responses from the 4-week period after the first mailing were included in the analysis.

The Hospital Anxiety and Depression Scale is a 14-item self-assessment scale for anxiety and depression (1821). Each item is scored from zero to three. Scores for both depression and anxiety range from 0 to 21, with a score of 11 or higher indicating caseness, a score of 8–10 rated as borderline, and scores below 8 indicating no disorder is present. The Icelandic version of the Hospital Anxiety and Depression Scale has been described elsewhere (22), and a recent review of its psychometric properties has found it to be both a valid and reliable instrument (23).

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Data Analysis

The software package used was BMDP (24). The tests used were t test, F test, and chi-square test. Double-sided p values and a significance level of 0.05 were used. Bonferroni correction was used for multiple tests. Analysis of covariance was applied to evaluate seasonal differences in mean Hospital Anxiety and Depression Scale scores for anxiety and depression. The mean values were adjusted for covariates, and the equality of slopes was tested. These analyses were also tested on logarithmic transformed Hospital Anxiety and Depression Scale scores; however, since the results were not appreciably different, they are not presented.

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Response Rates

Sixty-one letters were returned unopened. In 29 instances the code number had been removed, which precluded further analysis. Correcting for this, the total response rate was 57.8%. Questionnaires returned later than 4 weeks after the first mailing and questionnaires with less than four items answered for either anxiety or depression were excluded. Thus, a total of 2,262 (56.5%) questionnaires were included in the analysis (t1).

There were no significant differences in response rates across seasons for either the total sample or within groups. However, across all four mailings the response rate of the youngest group was lower than that of the oldest group (49.6% versus 60.9%; χ2=34, df=1, p<0.001). Response rates were also lower for men than for women (50.4% versus 62.3%; χ2=45, df=1, p<0.001). The response rate of subjects in the Reykjavík area was marginally lower than that of subjects in rural areas (54.9% versus 58.8%; χ2=4.6, df=1, p<0.03).

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Seasonal Variations

t2 shows the pattern of caseness. The rates of anxiety or depression did not show a statistically significant difference between winter and summer, either for actual cases, borderline cases, or for the two categories combined. Covariance analyses of Hospital Anxiety and Depression Scale scores showed 1) a nonsignificant gender effect for anxiety and depression, 2) anxiety scores were higher in the youngest age group (F=104.86, df=1, 2260, p<0.001), and 3) the depression scores were higher in the oldest age group (F=43.8, df=1, 2260, p<0.001) and in the rural population (F=19.1, df=1, 2260, p<0.001).

The average Hospital Anxiety and Depression Scale score for each season is shown in t3. The mean scores in winter were not higher than those in summer. In fact, after correcting for age and residency (urban versus rural), the only statistically significant difference in mean scores across seasons was that the depression scores in January were lower than those in all other seasons combined. However, this did not remain statistically significant after correcting for multiple tests. We examined specifically whether the youngest age group (20–35 years) had more seasonal variations than the older age groups. Even in this age group, the mean anxiety and depression scores in winter were not higher than those in summer (anxiety: 5.02 in January and 5.34 in July; depression: 2.48 in January and 2.94 in July).

This study would have had a 90% chance of detecting a significant (p<0.05) difference if the real underlying difference in mean scores between summer and winter had been more than 10% for anxiety and 12% for depression.

The absence of seasonal anxiety and depression found in the present study contrasts markedly with similar studies from other countries.

We are aware of 10 studies that have measured mental symptoms sequentially through the year. All had found seasonal variations with a peak for depression or anxiety in autumn or winter (t4). The pattern was complex in a few of these studies, but the general trend was quite consistent. Mersch and collaborators (32) mailed the Center for Epidemiologic Studies Depression Scale to a random sample in the Netherlands at different parts of the year. The average depression score was 8.5 in June and 12.4 in December. Murase and colleagues (14) had 242 Japanese who were residing in Stockholm fill out the Beck Depression Inventory in summer and winter. The scores were 65% higher in winter than in summer. In the United States, Harris and Dawson-Hughes (30) found 28% and 16% higher scores in winter than in summer for anxiety and depression, respectively. Two of the studies listed in t4 used the same questionnaires as the present study (21, 33). Furthermore, they were postal surveys with very similar study designs as ours. Both of these studies found higher depressive symptom scores in winter. Thus, our cross-sectional study is one of the few studies that finds no seasonal variations in either anxiety or depression, although it had many more participants than most, as shown in t4. It is remarkable that all of the studies listed in t4 find seasonal variations in mood even though they use a wide variety of instruments and different study designs.

This report is not a direct study of seasonal affective disorder but rather of seasonal mood variations. However, it is tempting to compare our results with those from a previous survey of seasonal affective disorder in the Icelandic population that used the Seasonal Pattern Assessment Questionnaire (11). That study found that Icelanders exhibit seasonal affective disorder and subsyndromal seasonal affective disorder, albeit at prevalences that were unexpectedly low given Iceland’s high latitude. By contrast, the present study, which was based upon much shorter recall periods than those of studies that used the Seasonal Pattern Assessment Questionnaire, detected no seasonal variations in anxiety and depression whatsoever in this population.

This discrepancy may have several explanations, such as the long-term recall bias toward magnifying symptoms (21) and the possibility of populations being artificially sensitized by the media to particular symptoms or disorders, such as seasonal affective disorder (34).

The low or even absent seasonality of depression is puzzling in itself and may reflect some unique features of the Icelandic population, possibly genetic in nature (10, 11, 35).

To address these issues a study that is longitudinally following a defined cohort, focusing on seasonal affective disorder, and using both the Hospital Anxiety and Depression Scale and the Seasonal Pattern Assessment Questionnaire is now in progress.

Received Feb. 12, 1998; revisions received Nov. 30, 1998, and May 28, 1999; accepted June 30, 1999. From Therapeia; the Research Forum and Department of Psychiatry, Ullevål Hospital, Oslo; and the Department of Physiology, University of Iceland, Reykjavík. Address reprint requests to Dr. Oskarsson, Therapeia, Sudurgata 12, 101 Reykjavík, Iceland; hoegni@mmedia.is (e-mail). Supported by two grants from the Icelandic Research Council. The authors thank Professor Tomas Helgason for his support and Helgi Sigvaldason for statistical advice.

       
Rosenthal NE, Genhart MJ, Sack DA, Wehr TA: Seasonal affective disorder and its relevance for the understanding and treatment of bulimia, in The Psychobiology of Bulimia. Edited by Hudson JI, Pope HG. Washington DC, American Psychiatric Press, 1987, pp 205–228
 
Kaspers S, Rogers SL, Yancey A, Schulz PM, Skwerer RG, Rosenthal NE: Phototherapy in individuals with and without subsyndromal seasonal affective disorder. Arch Gen Psychiatry  1989; 46:837–844
[PubMed]
 
Kasper S, Wehr TA, Bartko JJ, Gaist PA, Rosenthal NE: Epidemiological findings of seasonal changes in mood and behavior: a telephone survey of Montgomery County, Maryland. Arch Gen Psychiatry  1989; 46:823–833
[PubMed]
 
Rosen LN, Targum SD, Terman M, Bryant MJ, Hoffman H, Kasper SF, Hamovit JR, Docherty JP, Welch B, Rosenthal NE: Prevalence of seasonal affective disorders at four latitudes. Psychiatry Res  1990; 31:131–144
[PubMed]
[CrossRef]
 
Sakamoto K, Kamo T, Nakadaira S, Tamura A, Takahashi K: A nationwide survey of seasonal affective disorder at 53 outpatient university clinics in Japan. Acta Psychiatr Scand  1993; 87:258–265
[PubMed]
[CrossRef]
 
Carskadon MA, Acebo C: Parental reports of seasonal mood and behavior changes in children. J Am Acad Child Adolesc Psychiatry  1993; 32:264–269
[PubMed]
[CrossRef]
 
Williams RJ, Schmidt GG: Frequency of seasonal affective disorder among individuals seeking treatment at a northern Canadian mental health center. Psychiatry Res  1993; 46:41–45
[PubMed]
[CrossRef]
 
Potkin SG, Zetin M, Stamkovic V, Kripke D, Bunney WE Jr: Seasonal affective disorder: prevalence varies with latitude and climate. Clin Neuropharmacol 1986; 9(suppl):181–183
 
Booker JM, Hellekson CJ: Prevalence of seasonal affective disorder in Alaska. Am J Psychiatry 1992; 149:1176–  1182
 
Magnusson A, Axelsson J: The prevalence of seasonal affective disorder is low among descendants of Icelandic emigrants in Canada. Arch Gen Psychiatry  1993; 50:947–951
[PubMed]
 
Magnusson A, Stefansson JG: Prevalence of seasonal affective disorder in Iceland. Arch Gen Psychiatry  1993; 50:941–946
[PubMed]
 
Takahashi K, Asano Y, Kohsaka M, Okawa M, Sasaki M, Honsa Y, Higuchi T, Yamazaki J, Ishizuka Y, Kawaguchi K: Multi-center study of seasonal affective disorder in Japan: a preliminary report. J Affect Disord  1991; 21:57–65
[PubMed]
[CrossRef]
 
Ozaki N, Ono Y, Ito A, Rosenthal NE: Prevalence of seasonal difficulties in mood and behavior among Japanese civil servants. Am J Psychiatry 1995; 152:1225–  1227
 
Murase S, Murase S, Kitabatake M, Yamauchi T, Mathe AA: Seasonal mood variations among Japanese residents of Stockholm. Acta Psychiatr Scand  1995; 92:51–55
[PubMed]
[CrossRef]
 
Axelsson J, Stefansson JG, Magnusson A, Sigvaldason H, Karlsson MM: Seasonal affective disorders: the relevance of Icelandic and Icelandic-Canadian evidence to aetiological hypotheses, in Program and Abstracts, Sixth Meeting of the Society for Research on Biological Rhythms. Charlottesville, Va, SRBR, 1998, p 144
 
Bromet EJ, Dunn LO, Connel NM, Dew MA, Schulberg HC: Long-term reliability of diagnosing lifetime major depression in a community sample. Arch Gen Psychiatry  1986; 43:435–440
[PubMed]
 
Wicki W, Angst J, Merikangas KR: The Zurich Study, XIV: epidemiology of seasonal depression. Eur Arch Psychiatry Clin Neurosci  1992; 241:301–306
[PubMed]
[CrossRef]
 
Zigmond AS, Snaith RP: The Hospital Anxiety and Depression Scale. Acta Psychiatr Scand  1983; 67:361–370
[PubMed]
[CrossRef]
 
Wilkinson MJ, Barczak P: Psychiatric screening in general practice: comparison of the General Health Questionnaire and the Hospital Anxiety and Depression Scale. J R Coll Gen Pract  1988; 38:311–313
[PubMed]
 
Aylard PR, Gooding JH, McKenna PJ, Snaith RP: A validation study of three anxiety and depression self-assessment scales. J Psychosom Res  1987; 31:261–268
[PubMed]
[CrossRef]
 
Nayyar K, Cochrane R: Seasonal changes in affective state measured prospectively and retrospectively. Br J Psychiatry  1996; 168:627–632
[PubMed]
[CrossRef]
 
Schaaber U, Smari J, Oskarsson H: Comparison of the hospital anxiety and depression scale with other rating scales. Nord Psykiatr Tidsskr  1990; 44:507–512
[CrossRef]
 
Herrmann C: International experience with the Hospital Anxiety and Depression Scale—a review of validation data and clinical results. J Psychosom Res  1997; 42:17–41
[PubMed]
[CrossRef]
 
Dixon WJ (ed): BMDP Statistical Software Manual, 1990 ed, vols 1 and 2. Berkeley, University of California Press, 1990
 
Lacoste V, Wirz JA: Seasonality in personality dimensions. Psychiatry Res  1987; 21:181–183
[PubMed]
[CrossRef]
 
Lacoste V, Wirz JA: Seasonal variation in normal subjects: an update of variables current in depression research, in Seasonal Affective Disorders and Phototherapy. Edited by Rosenthal NE, Blehar MC. New York, Guilford Press, 1989, pp 167–229
 
Terman M, Botticelli SR, Link BG, Quitkin FM, Hardin TE, Rosenthal NE: Seasonal symptom patterns in New York: patients and population, in Seasonal Affective Disorders. Edited by Thompson C, Silverstone T. London, CNS, 1989, pp 77–95
 
Haggag A, Eklund B, Linaker O, Gotestam KG: Seasonal mood variation: an epidemiological study in northern Norway. Acta Psychiatr Scand  1990; 81:141–145
[PubMed]
[CrossRef]
 
Schlager D, Schwartz JE, Bromet EJ: Seasonal variations of current symptoms in a healthy population. Br J Psychiatry  1993; 163:322–326
[PubMed]
[CrossRef]
 
Harris S, Dawson-Hughes B: Seasonal mood changes in 250 normal women. Psychiatry Res  1993; 49:77–87
[PubMed]
[CrossRef]
 
Nayha S, Vaisanen E, Hassi J: Season and mental illness in an Arctic area of northern Finland. Acta Psychiatr Scand Suppl  1994; 377:46–49
[PubMed]
 
Mersch PPA, Middendorp H, Bouhuys AL, Beersma DGM, Van den Hoofdakker RH: The prevalence of seasonal affective disorder in the Netherlands. Acta Neuropsychiatr  1995; 7:47–49
 
Suhail K, Cochrane R: Seasonal changes in affective state in samples of Asian and white women. Soc Psychiatry Psychiatr Epidemiol  1997; 32:149–157
[PubMed]
 
Hotopf M: Seasonal affective disorder, environmental hypersensitivity and somatization. Br J Psychiatry  1994; 164:246–248
[PubMed]
[CrossRef]
 
Stefansson JG, Magnusson A, Karlsson MM, Axelsson J: Low prevalence of SAD in Icelanders and Canadians of Icelandic descent. Arctic Med Res 1994; 53(suppl 2):491–492
 
+

References

Rosenthal NE, Genhart MJ, Sack DA, Wehr TA: Seasonal affective disorder and its relevance for the understanding and treatment of bulimia, in The Psychobiology of Bulimia. Edited by Hudson JI, Pope HG. Washington DC, American Psychiatric Press, 1987, pp 205–228
 
Kaspers S, Rogers SL, Yancey A, Schulz PM, Skwerer RG, Rosenthal NE: Phototherapy in individuals with and without subsyndromal seasonal affective disorder. Arch Gen Psychiatry  1989; 46:837–844
[PubMed]
 
Kasper S, Wehr TA, Bartko JJ, Gaist PA, Rosenthal NE: Epidemiological findings of seasonal changes in mood and behavior: a telephone survey of Montgomery County, Maryland. Arch Gen Psychiatry  1989; 46:823–833
[PubMed]
 
Rosen LN, Targum SD, Terman M, Bryant MJ, Hoffman H, Kasper SF, Hamovit JR, Docherty JP, Welch B, Rosenthal NE: Prevalence of seasonal affective disorders at four latitudes. Psychiatry Res  1990; 31:131–144
[PubMed]
[CrossRef]
 
Sakamoto K, Kamo T, Nakadaira S, Tamura A, Takahashi K: A nationwide survey of seasonal affective disorder at 53 outpatient university clinics in Japan. Acta Psychiatr Scand  1993; 87:258–265
[PubMed]
[CrossRef]
 
Carskadon MA, Acebo C: Parental reports of seasonal mood and behavior changes in children. J Am Acad Child Adolesc Psychiatry  1993; 32:264–269
[PubMed]
[CrossRef]
 
Williams RJ, Schmidt GG: Frequency of seasonal affective disorder among individuals seeking treatment at a northern Canadian mental health center. Psychiatry Res  1993; 46:41–45
[PubMed]
[CrossRef]
 
Potkin SG, Zetin M, Stamkovic V, Kripke D, Bunney WE Jr: Seasonal affective disorder: prevalence varies with latitude and climate. Clin Neuropharmacol 1986; 9(suppl):181–183
 
Booker JM, Hellekson CJ: Prevalence of seasonal affective disorder in Alaska. Am J Psychiatry 1992; 149:1176–  1182
 
Magnusson A, Axelsson J: The prevalence of seasonal affective disorder is low among descendants of Icelandic emigrants in Canada. Arch Gen Psychiatry  1993; 50:947–951
[PubMed]
 
Magnusson A, Stefansson JG: Prevalence of seasonal affective disorder in Iceland. Arch Gen Psychiatry  1993; 50:941–946
[PubMed]
 
Takahashi K, Asano Y, Kohsaka M, Okawa M, Sasaki M, Honsa Y, Higuchi T, Yamazaki J, Ishizuka Y, Kawaguchi K: Multi-center study of seasonal affective disorder in Japan: a preliminary report. J Affect Disord  1991; 21:57–65
[PubMed]
[CrossRef]
 
Ozaki N, Ono Y, Ito A, Rosenthal NE: Prevalence of seasonal difficulties in mood and behavior among Japanese civil servants. Am J Psychiatry 1995; 152:1225–  1227
 
Murase S, Murase S, Kitabatake M, Yamauchi T, Mathe AA: Seasonal mood variations among Japanese residents of Stockholm. Acta Psychiatr Scand  1995; 92:51–55
[PubMed]
[CrossRef]
 
Axelsson J, Stefansson JG, Magnusson A, Sigvaldason H, Karlsson MM: Seasonal affective disorders: the relevance of Icelandic and Icelandic-Canadian evidence to aetiological hypotheses, in Program and Abstracts, Sixth Meeting of the Society for Research on Biological Rhythms. Charlottesville, Va, SRBR, 1998, p 144
 
Bromet EJ, Dunn LO, Connel NM, Dew MA, Schulberg HC: Long-term reliability of diagnosing lifetime major depression in a community sample. Arch Gen Psychiatry  1986; 43:435–440
[PubMed]
 
Wicki W, Angst J, Merikangas KR: The Zurich Study, XIV: epidemiology of seasonal depression. Eur Arch Psychiatry Clin Neurosci  1992; 241:301–306
[PubMed]
[CrossRef]
 
Zigmond AS, Snaith RP: The Hospital Anxiety and Depression Scale. Acta Psychiatr Scand  1983; 67:361–370
[PubMed]
[CrossRef]
 
Wilkinson MJ, Barczak P: Psychiatric screening in general practice: comparison of the General Health Questionnaire and the Hospital Anxiety and Depression Scale. J R Coll Gen Pract  1988; 38:311–313
[PubMed]
 
Aylard PR, Gooding JH, McKenna PJ, Snaith RP: A validation study of three anxiety and depression self-assessment scales. J Psychosom Res  1987; 31:261–268
[PubMed]
[CrossRef]
 
Nayyar K, Cochrane R: Seasonal changes in affective state measured prospectively and retrospectively. Br J Psychiatry  1996; 168:627–632
[PubMed]
[CrossRef]
 
Schaaber U, Smari J, Oskarsson H: Comparison of the hospital anxiety and depression scale with other rating scales. Nord Psykiatr Tidsskr  1990; 44:507–512
[CrossRef]
 
Herrmann C: International experience with the Hospital Anxiety and Depression Scale—a review of validation data and clinical results. J Psychosom Res  1997; 42:17–41
[PubMed]
[CrossRef]
 
Dixon WJ (ed): BMDP Statistical Software Manual, 1990 ed, vols 1 and 2. Berkeley, University of California Press, 1990
 
Lacoste V, Wirz JA: Seasonality in personality dimensions. Psychiatry Res  1987; 21:181–183
[PubMed]
[CrossRef]
 
Lacoste V, Wirz JA: Seasonal variation in normal subjects: an update of variables current in depression research, in Seasonal Affective Disorders and Phototherapy. Edited by Rosenthal NE, Blehar MC. New York, Guilford Press, 1989, pp 167–229
 
Terman M, Botticelli SR, Link BG, Quitkin FM, Hardin TE, Rosenthal NE: Seasonal symptom patterns in New York: patients and population, in Seasonal Affective Disorders. Edited by Thompson C, Silverstone T. London, CNS, 1989, pp 77–95
 
Haggag A, Eklund B, Linaker O, Gotestam KG: Seasonal mood variation: an epidemiological study in northern Norway. Acta Psychiatr Scand  1990; 81:141–145
[PubMed]
[CrossRef]
 
Schlager D, Schwartz JE, Bromet EJ: Seasonal variations of current symptoms in a healthy population. Br J Psychiatry  1993; 163:322–326
[PubMed]
[CrossRef]
 
Harris S, Dawson-Hughes B: Seasonal mood changes in 250 normal women. Psychiatry Res  1993; 49:77–87
[PubMed]
[CrossRef]
 
Nayha S, Vaisanen E, Hassi J: Season and mental illness in an Arctic area of northern Finland. Acta Psychiatr Scand Suppl  1994; 377:46–49
[PubMed]
 
Mersch PPA, Middendorp H, Bouhuys AL, Beersma DGM, Van den Hoofdakker RH: The prevalence of seasonal affective disorder in the Netherlands. Acta Neuropsychiatr  1995; 7:47–49
 
Suhail K, Cochrane R: Seasonal changes in affective state in samples of Asian and white women. Soc Psychiatry Psychiatr Epidemiol  1997; 32:149–157
[PubMed]
 
Hotopf M: Seasonal affective disorder, environmental hypersensitivity and somatization. Br J Psychiatry  1994; 164:246–248
[PubMed]
[CrossRef]
 
Stefansson JG, Magnusson A, Karlsson MM, Axelsson J: Low prevalence of SAD in Icelanders and Canadians of Icelandic descent. Arctic Med Res 1994; 53(suppl 2):491–492
 
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