SAD latitude UVB etc
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LIGHT THERAPY http://www.aafp.org/afp/2006/1101/p1521.html   2006
Among susceptible persons, decreased seasonal exposure to light may mediate SAD through phase shifts in circadian rhythms, with resulting alterations in several aspects of serotonin metabolism. Thus, light replacement has been the most widely studied treatment for SAD.20 In a review of studies of light therapy, an average dosage of 2,500 lux daily for one week was superior to placebo, as indicated by improvements on a depression rating scale.21 The dosage most often found to be effective is 5,000 lux per day, given as 2,500 lux for two hours or 10,000 lux for 30 minutes.22 A recent meta-analysis of 23 studies of light therapy found that the odds ratio for remission was 2.9 (95% confidence interval, 1.6 to 5.4); this ratio is similar to those of many pharmaceutical treatments for depression.23 Like drug therapy for depression, light therapy carries some risk of precipitating mania.24
Light therapy generally is most effective when administered earlier in the day.21,25,26 Early morning light therapy regulates the circadian pattern of melatonin secretion, whereas the use of light in the evening delays the normal melatonin phase shift.27
To ensure adequate response, patients should be treated with light therapy units that are specifically designed to treat SAD. Units that are not specifically designed for SAD treatment may not provide adequate brightness and may not have appropriate ultraviolet light filtration.22
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J Affect Disord. 1999 Apr;53(1):35-48.
Seasonal affective disorder and latitude: a review of the literature.
Mersch PP, Middendorp HM, Bouhuys AL, Beersma DG, van den Hoofdakker RH.
Department of Biological Psychiatry, University Hospital Groningen, The Netherlands.
BACKGROUND: The aim of the study is to investigate the relationship between the prevalence of SAD and latitude. METHODS: An overview of the epidemiological literature on the prevalence of SAD is given and studies relevant for the latitudinal dependency of prevalence will be analyzed and discussed. RESULTS: The mean prevalence of SAD is two times higher in North America compared to Europe. Over all prevalence studies, the correlation between prevalence and latitude was not significant. A significant positive correlation was found between prevalence and latitude in North America. For Europe there was a trend in the same direction. 
CONCLUSIONS: The influence of latitude on prevalence seems to be small and other factors like climate, genetic vulnerability and social-cultural context can be expected to play a more important role. Additional controlled studies taking these factors into account are necessary to identify their influence.
PMID: 10363665 [PubMed - indexed for MEDLINE]
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Review of book   http://ajp.psychiatryonline.org/cgi/content/full/160/7/1364-a 
Seasonal Affective Disorder: Practice and Research
Edited by Timo Partonen and Andres Magnusson. New York, Oxford University Press, 2001, 311 pp., $89.00.
KATHRYN J. EDNIE, M.D.
Ann Arbor, Mich.
This book on seasonal affective disorder arrived for review on one of those gun-metal gray, short November days that stretch into long gray November weeks, weather predictable for Michigan in late fall. On the cover is a lamplighter in the fog of a monochromic picture, so like the endlessly dark days of late fall and winter in the North. Those who might question the existence of this syndrome have only to spend some time in this continually cold and drab climate or, perhaps, more happily, can review the studies cited in this book and make a decision about the syndrome from a sunny location.
Seasonal affective disorder is a subtype of depression characterized by depressed mood, lack of energy, hypersomnia, craving for carbohydrates, overeating, and weight gain with a pattern of seasonal change (1). A more prevalent but attenuated form of the disorder has been labeled "subsyndromal seasonal affective disorder." Although the effect of the seasons on mood has been mentioned throughout the ages, light and its effect on seasonal variations in mood have been subjected to much more intense study since the early 1980s. An increased focus on exploration of circadian rhythms, along with the discovery of the effectiveness of bright light treatment and its subsequent commercial availability, has augmented the international study of this disorder. The syndrome is often explored in the popular press, leading to substantial public awareness. Perhaps some of the interest is due to the fact that there is anecdotal as well as research evidence that seasonal variations in mood may be present in the normal population.
The authors of this edited volume reside in Helsinki and Oslo, highlighting an issue addressed in the chapters on weather and on the latitude gradient in the prevalence of the rates of seasonal affective disorder. (Interestingly, when latitude-prevalence studies are summarized, there is a balance between studies confirming and studies rejecting the latitude-prevalence hypothesis [p. 134].) Partonen and Magnusson have condensed the expanding literature on this syndrome into a multichapter volume that provides a comprehensive review of seasonal affective disorder, with a particular focus on the winter subtype.
The volume is divided into two sections. The first highlights clinical issues, including symptoms and treatment, and is written for a wide range of health care and mental health professionals. The second section is more academic and research focused. All of the chapters in the book are heavily referenced and cite the latest international literature. The book is weighted toward review of the research in areas such as clinical symptoms (sleep, for example), neuroimaging, and biological clock genes. Throughout the book there is an objective view of the syndrome, including chapters about the controversial issues of the placebo effect of light therapy and the timing of successful light treatment. Explored from several different perspectives are the competing explanatory models for seasonal affective disorder—the phase shift hypothesis versus the melatonin hypothesis. Studies of response to light treatment have failed to resolve the conflict between these two hypotheses. The fact that morning light is more effective than evening light supports the phase shift hypothesis. On the other hand, the fact that evening light is more effective than control conditions supports the melatonin argument (2, 3).
Like studies of the latitude-prevalence hypothesis, the examination of outcomes of studies has continued to raise more questions rather than providing a clear answer to many of the controversial issues regarding seasonal affective disorder. After reviewing this volume I had more of a sense of the uncertainty of the field than when I began. I can appreciate the excitement that research in this field generates because many questions remain to be answered. This book gives a good overview of the remaining controversial issues and is intellectually honest and unbiased in its approach. In summary, it is a fascinating review of research in this field that leaves the reader with an understanding of the issues and eager to see how they will ultimately be resolved.
References
   1. Allen JM, Lam RW, Remick RA, Sadovnick AD: Depressive symptoms and family history in seasonal and nonseasonal mood disorders. Am J Psychiatry 1993; 150:443-448[Abstract/Free Full Text]
   2. Lewy AJ, Bauer VK, Cutler NL, Sack RL, Ahmed S, Thomas KH, Blood ML, Jackson JM: Morning vs evening light treatment of patients with winter depression. Arch Gen Psychiatry 1998; 55:890-896[Abstract/Free Full Text]
   3. Terman JS, Terman M, Lo ES, Cooper TB: Circadian time of morning light administration and therapeutic response in winter depression. Arch Gen Psychiatry 2001; 58:69-75[Abstract/Free Full Text]
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Epidemiol Psichiatr Soc. 2009 Oct-Dec;18(4):336-43.
Seasonal affective disorder and latitude of living.
Brancaleoni G, Nikitenkova E, Grassi L, Hansen V.
Department of Clinical Psychiatry, Institute of Clinical Medicine, Faculty of Medicine, University of Tromsø, and Psychiatric Department, University Hospital North, Norway.
AIM: Since the importance of latitude of living for the prevalence of Seasonal Affective Disorder (SAD) is unclear, the study aims to test the latitude hypothesis by comparing SAD in two rather similar groups of students living at latitudes far apart. METHODS: Two groups of students, 199 in Tromsø, Norway (690 N) and 188 in Ferrara, Italy (440 N) were asked to fill in the Seasonal Pattern Assessment Questionnaire. RESULTS: Global Seasonality score (GS-score) was significantly higher in Italian than in Norwegian students, in females and in students with sleeping-problems. Norwegian students had significantly higher SAD prevalence in winter and in spring. {Probably due to low vitamin D} Most people in both countries felt worst in October and November, and the prevalence of Autumn SAD was not significantly different between the two countries. CONCLUSIONS: The hypothesis that SAD is linked to amount of environmental light and latitude of living was not supported.
PMID: 20170049 [PubMed - in process]
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Bluer skies in Iceland might be cause of less SAD in Iceland ( but might also be evolution)
Int J Circumpolar Health. 2004 May;63(2):145-56.
Chromaticity of daylight: is the spectral composition of daylight an aetiological element in winter depression?
Axelsson J, Ragnarsdóttir S, Pind J, Sigbjörnsson R.
Institute of Physiology, University of Iceland, Reykjavik, Iceland.
OBJECTIVES: Surveys on winter depression in Iceland indicate a significantly lower prevalence rate of winter SAD than expected according to Iceland's latitude. Research into daylight availability in Iceland failed to reveal factors contributing to higher average daylight availability than predicted by latitude. In view of the well-known healing effects of bright light treatment, we propose that properties of daylight other than daylight availability may ease the symptoms of winter depression. 
METHOD: We analysed the spectral composition of daylight in Iceland as expressed by its chromaticity and assessed its seasonal and diurnal variations. The colorimetric properties of daylight during the year 1998 are dealt with in detail. Perception of daylight is modelled, applying the chromaticity model of MacLeod and Boynton along with environmental data on spectral irradiance recorded on location at 64 degrees 8.8' N and 21 degrees 55.8' W in Reykjavik, Iceland, and recently published data on cone fundamentals by Stockman and Sharpe. 
RESULTS: The main finding is that blue hue dominates the colour of the sky, with high correlated colour temperature, without significant seasonal variations. Diurnal variations are, however, observed. Furthermore, significant deviation from 'standard' sky is detected. 
CONCLUSIONS: It is not known whether the observed chromaticity of daylight is a significant factor in explaining the unexpectedly low prevalence rate of seasonal affective disorder in Iceland.
PMID: 15253481 [PubMed - indexed for MEDLINE]
Note: they did not appear to measure the UV, nor did they look at the reflection off of snow
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Clipped from http://priory.com/psych/SAD.htm 
Early SAD studies indicated that most patients met criteria for bipolar II disorder (depressive and hypomanic episodes), while only a small proportion had bipolar I disorder (depressive and full manic episodes), or unipolar depression8. More recent work, however, has found that the majority of patients to have unipolar depression, with a substantial minority of patients having bipolar II disorder and very few having bipolar I disorder9. This discrepancy could possibly be explained because of the use of different sets or criteria or due to different climatic conditions between studies.
The prevalence of SAD has been estimated to range from 0 to 9.7% depending on the population that is being sampled5. Women are approximately twice as likely to suffer from SAD than men, { women have more fat and less vitamin D in the blood – wonder about SAD for elderly, dark skin, obese, etc} which is comparable to gender differences reported in non-seasonal depression although they may reflect bias in sampling10. According to a study, the mean age of onset in SAD appears to be in the thirties11.
Though the geographical distribution of SAD has not been rigorously studied there is evidence to suggest that the prevalence of SAD and S-SAD increases with an increase in northern latitude1. This led to the view that SAD is linked to the hours of daylight that people are exposed to at different latitudes, where the further north a person lives the fewer hours of sunlight exposure per day. Research in different areas of the USA found there was a correlation between latitude and prevalence of SAD (Table 4).
Table 4: The prevalence of SAD and S-SAD in a survey carried out in the USA12

SAD = seasonal affective disorder; S-SAD = subsyndromal SAD.
However, when compared with the results above, several studies have found lower rates of SAD in Iceland and Manitoba which are located at 63-67o north and 50o north respectively13. Even among descendants of Icelandic emigrants in Canada, the prevalence was low suggesting genetic adaptation might play an important role14. This finding seems to emphasize the importance of genetic, psychosocial, cultural and ethnic factors in the development of SAD.
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