Shortly before the winter solstice near Basel, Switzerland, photo courtesy of Dominik Wunderlin.

Michael Terman PhD is president of the Center for Environmental Therapeutics and a professor of clinical psychology in psychiatry at Columbia University.

Shortly before the winter solstice near Basel, Switzerland. Photo courtesy of Dominik Wunderlin, Basel

December 2020. With the winter solstice upon us, the season for winter depression approaches its peak.  We know from many studies in the U.S., Europe, Russia and Japan, that vulnerability to repeated major depressive episodes (SAD) between November and February is present in around 2% to 5% of the population — study estimates vary quite a lot —  with around three times as many suffering from the winter blues. But the numbers are not that simple. Where you live on planet earth, and when the sun rises, are important environmental factors that have yet to be widely addressed in terms of susceptibility to SAD.

Some years ago, my research group carried out a web-based study in over 5000 respondents to a set of validated chronotype, depression, and seasonality questionnaires. Although anonymous, we knew their ZIP codes, and could locate their latitude (from 28° to 47 °N) and longitude (from 67° to 125 °W) within the four continental U.S. time zones.

To make our point about location of residence and susceptibility to SAD, let us first consider the graphic of sunrise across the U.S. on the winter solstice (a beautiful creation by George Musa, PhD of the New York State Psychiatric Institute).


The display can be explained as follows: Efficient humans decided more than 100 years ago that the world should be chopped up into pragmatic time zones, to keep the trains running on time. Some time zones are fatter than others, and most are arbitrary, rarely related to state boundaries and often zigzagging along mountain ranges and rivers. The time zones do not represent the reality of the solar day, as it is anchored to sunrise at dawn. At any latitude within a given time zone, the sun rises progressively later from the east to west (blue to yellow to red), and then jumps an hour earlier when the time zone is crossed (suddenly blue again). Dawn is also earlier in the south of a given time zone, and later in the north. In numbers, that means sunrise in the north occurs 60 minutes later than in the south, and sunrise in the west is 72 minutes later than at the eastern boundary of a time zone (across 18° longitude). These are not trivial differences for our biological clock and sleep timing. The dawn signal is crucial for our biological clock to wake us up in synchrony with the day-night cycle. The progressively later clock time of sunrise in winter, is thought to be depressogenic. And our study showed that the incidence of SAD indeed follows this pattern — highest in regions when dawn is late.


The relative occurrence of SAD with increasing U.S. latitude


The results can be plotted as a positive latitude cline of SAD frequency. A very low occurrence of SAD in Florida where dawn is early, shifts gradually higher to New York (40 °N) where the sun rises half an hour later — but it surprisingly ceilings out farther north. Most earlier SAD surveys had shown no convincing latitude effect when sampling individuals in European and Canadian regions above 38 °N, corresponding to the U.S. ceiling. Additionally, it is known that native peoples in the Arctic — whether the Saami in Finland, the Tschuktschi in Siberia, or the Inuit in Canada —  have evolved a resistance to this debilitating mood disorder which would strongly affect their chances of survival during intemperate winters. 







The relative occurrence of SAD at increasing longitude within time zones (above 38 °N latitude)



There is however, a second critical factor: longitude.  We found in the latitude range above 38 °N that the farther west you live within a time zone, the higher the frequency of SAD. Why? Because the delayed clock time of sunrise in the west of a time zone is more likely to overshoot wake-up time during the winter work week. This exaggerates the incidence of winter depression, raises the proportion of people with clinically significant seasonality, and promotes atypical neurovegetative symptoms, including increased sleeping, daytime fatigue, carbohydrate craving and weight gain. Cross over the time zone border and lo! the sun suddenly rises an hour earlier . . . and there is less SAD. 







So, the latitude factor suggests that individuals prone to winter depression would benefit by moving well below the Mason-Dixon line      (39° 43’). Indeed, several of our patients have made the move, to their benefit. Many may be motivated primarily to escape the cold farther north — the obvious explanation — but those who responded to light therapy in clinical trials were also attuned to the subtler circadian factor of delayed sunrise.

Alternately — or even additionally — the data indicate a move to the east of a time zone to protect SAD vulnerability. Such a major migration is obviously not realistic, except for those who can relocate a few more miles west where the sun rises an hour earlier. We are not necessarily recommending such travel; there are many other reasons it might not work out. But these research observations should instill an awareness of the importance of daylight, in particular the dawn signal, to sync your inner clock and mood. Try going outdoors in the morning! Even with the delayed sunrise in winter, being outside provides high intensity daylight. A functional solution for those indoors is a dawn simulator set to an early wake-up time, a classical light box in the morning, or both.


These data were first presented at the 2005 annual meeting of the Society for Light Treatment and Biological Rhythms in Eindhoven, The Netherlands: Incidence Winter Depression Varies Within Time Zones, by TM White, M Terman, GJ Musa, and DH Avery.