With daylight savings time in full swing and the winter solstice nearing, many individuals -- particularly those in northern latitudes -- experience changes in their well-being. Data indicate that women may suffer more from seasonal alterations in health and behavior than men. These seasonal changes involve a wide range of issues from mood changes, to decreases in fertility, to changes in biomarkers and behaviors linked to cardiovascular risk. But without a better understanding of the effects of season and light exposure on health, it will be difficult to limit risk and find treatments for both women and men.
Perhaps the best-known seasonal change in health is seasonal affective disorder or SAD, sometimes also called Winter Depression. According to the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) , SAD is a form of major depressive disorder that is characterized by a cyclical pattern of depressive symptoms that appear during the same season every year -- typically winter -- with a return to normal mood when the season changes.
Most SAD patients report depressed or sad moods, decreased energy, increased sleep, decreased physical activity, and decreased interest in social interaction and their usual activities. SAD patients also may experience a craving for certain foods, such as carbohydrates, along with weight gain and mood fluctuations throughout the day.
Sex differences in SAD start around puberty. For instance, after age 13 girls report more seasonal depression symptoms than boys the same age  and teen girls past puberty are more than twice as likely to have a diagnosis of SAD than girls who have not passed this maturational milestone .
Studies show that by adulthood, women are about four times more likely to suffer from SAD symptoms than men . Interestingly, the difference in SAD between women and men is strongest during the reproductive years. At menopause, that gender gap begins to close and at least one study suggests that this decrease in susceptibility to SAD is linked to waning of the monthly cycling of the female hormones estrogen and progesterone that occurs in menopausal women .
Similarly, seasonal changes in fertility may occur when neuroendocrine signals that convey information about the short days and long nights interact with female hormones that support reproduction. Human pregnancies tend to begin in the spring and fall, rather than winter or summer . Research that has examined seasonality with respect to conception in women using assisted reproductive technology, such as in vitro fertilization (IVF), offers important clues to the role that long nights might play in human fertility.
In theory, so many factors are controlled in IVF that different conception rates might not be expected by season. Nevertheless, a study of 305 women undergoing IVF found that the highest fertilization rates occurred during the spring and the lowest occurred in autumn. In addition, the study showed that fertility rate was closely related to hours of daylight at the time of conception, and not to other environmental variables like temperature or humidity . A related study of almost 2,000 women using the assisted reproductive treatment known as intracytoplasmic sperm injection (ICSI) also showed a higher fertilization rate in the spring than during any other season .
Heart disease is the leading cause of death in American women  and there is a peak in deaths from heart disease in the winter among both sexes . In women, this is associated with seasonal differences in body chemicals that may increase risk, or serve as markers of increased risk, for heart disease. For instance, C-reactive protein (CRP), a biomarker of inflammation that is associated with heart disease, rises in winter with women experiencing increases that average twice that of men .
Women are also more likely than men to show clinically-significant decreases in Vitamin D during the winter months . Vitamin D affects many body systems and Vitamin D deficiency is a risk factor for heart attacks, strokes and other conditions associated with heart disease, like diabetes and high blood pressure. Finally, there is evidence that females have lower physical activity than males during winter months .
Despite evidence for changes in women's health in winter, the biology and physiology that contribute to these seasonal sex differences are not well understood. One clue comes from a preliminary study that showed that women's biological clocks are much more likely to respond to seasonal changes in the light-dark cycle than men's clocks . In this study, the timing of melatonin release changed by 80 minutes from summer to winter in women, whereas men differed by less than 20 minutes across seasons.
Studies are underway to reveal more information about how the long winter nights affect human health. And since many seasonal changes appear to be worse for women than men, it is vitally important that the studies not only include women, but that they also include enough women to examine for differences between men and women.
By Katherine M. Sharkey, MD, PhD, FAASM, Member of the SWHR Network on Sleep and Assistant Professor of Medicine and Psychiatry and Human Behavior, Warren Alpert Medical School of Brown University and Rhode Island Hospital
1) American Psychiatric Association. The Diagnostic and Statistical Manual of Mental Disorders. 5th ed. Washington DC, 2013.
2) Tonetti L, Barbato G, Fabbri M, Adan A, Natale V. Mood seasonality: a cross-sectional study of subjects aged between 10 and 25 years. J Affect Disord. 2007;97:155-60.
3) Swedo SE, Pleeter JD, Richter DM, Hoffman CL, Allen AJ, Hamburger SD, et al. Rates of seasonal affective disorder in children and adolescents. Am J Psychiatry. 1995;152:1016-9.
4) Rosen LN, Targum SD, Terman M, Bryant MJ, Hoffman H, Kasper SF, et al. Prevalence of seasonal affective disorder at four latitudes. Psychiatry Res. 1990;31:131-44.
5) 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;46:823-33.
6) Wehr TA. Photoperiodism in humans and other primates: evidence and implications. J Biol Rhythms. 2001;16:348-64.
7) Rojansky N, Benshushan A, Meirsdorf S, Lewin A, Laufer N, Safran A. Seasonal variability in fertilization and embryo quality rates in women undergoing IVF. Fertility and sterility. 2000;74:476-81.
8) Braga DP, Setti A, Figueira Rde C, Iaconelli A, Jr., Borges E, Jr. Seasonal variability in the fertilization rate of women undergoing assisted reproduction treatments. Gynecological endocrinology : the official journal of the International Society of Gynecological Endocrinology. 2012;28:549-52.
9) Kochanek K, Xu J, Murphy S, Miniño A, Kung H. Deaths: final data for 2009. National vital statistics reports. 2011;60.
10) Pell JP, Cobbe SM. Seasonal variations in coronary heart disease. QJM : monthly journal of the Association of Physicians. 1999;92:689-96.
11) Chiriboga DE, Ma Y, Li W, Stanek EJ, 3rd, Hebert JR, Merriam PA, et al. Seasonal and sex variation of high-sensitivity C-reactive protein in healthy adults: a longitudinal study. Clinical chemistry. 2009;55:313-21.
12) Carnevale V, Modoni S, Pileri M, Di Giorgio A, Chiodini I, Minisola S, et al. Longitudinal evaluation of vitamin D status in healthy subjects from southern Italy: seasonal and gender differences. Osteoporosis international : a journal established as result of cooperation between the European Foundation for Osteoporosis and the National Osteoporosis Foundation of the USA. 2001;12:1026-30.
13) Gracia-Marco L, Ortega FB, Ruiz JR, Williams CA, Hagstromer M, Manios Y, et al. Seasonal variation in physical activity and sedentary time in different European regions. The HELENA study. Journal of sports sciences. 2013;31:1831-40.
14) Cain S, Vlassac I, Gooley J, Rahman SA, Van Reen E, Rueger M, et al. Sex differences in seasonal timing of the circadian clock in humans. Society for Research on Biological Rhythms Abstracts. 2012;13:90.