Under the weather

November 19th, 2024

Written by: Lisa Wooldridge

Ahh, November. The air is getting crisper, the days are getting shorter, and the trees become bare. Winter is coming. Is it your favorite time of year, or is it the winter of your discontent?

Many people are their grumpiest during the winter. At the extreme end of the scale, some people experience Seasonal Affective Disorder, a type of depression that patients experience primarily during only certain seasons.¹ Symptoms of other mood disorders, like Major Depressive Disorder, Bipolar Disorder, and Generalized Anxiety Disorder, are also generally at their worst during the winter.^2,3  Clearly seasons, and the weather they bring, can have profound effects on our mood. Years of research has found numerous reasons why. Here are just a few aspects of the weather that can affect our mood.

Sunlight and circadian rhythms

Sunlight plays an important role in biology, and weather can affect how much of it we get. As the winter equinox approaches, we get fewer and fewer hours of sunlight each day. As we transition back towards spring, we get more and more sunlight each day. In addition, stormy weather any time of year means clouds and dark skies. These daily and seasonal changes in sunlight can have a major impact on our mood.

Our brains use sunlight to judge the time of day. If you’ve ever experienced jetlag, you know how disruptive it is when your body doesn’t know what time of day it is. Our body keeps track of the time of day with circadian rhythms, which are cycling patterns of biological processes or behaviors that peak and trough approximately once per day. Sleepiness provides a familiar example. If left to our own devices, we tend to get very sleepy around the same time every night (a sleepiness peak), and naturally wake up around the same time every day (a sleepiness trough). As a result, we get tired early for the first couple of days when we fly east across time zones, and wake up before dawn for a couple of days when we fly west.

Sleepiness is not the only biological rhythm – there are many such biological processes and behaviors, which is why jetlag can leave us feeling so off-balance. In complete darkness, all of these processes still have their defining rhythms,  but each process does so on its own timeline – and most don’t peak precisely every 24 hours. As a result, in complete darkness, each process or behavior peaks at a different time each day. Light’s key role, then, is to synchronize all these processes – to tell our body what time it is.

How does light get all our circadian rhythms to line up? When our eyes detect light, they  send a message to a part of the brain called the suprachiasmatic nucleus.⁴ As daylight fades to evening, and as dawn breaks, subtle changes in light change the strength of the signal from the eyes to the suprachiasmatic nucleus, which uses that information to calculate the time of day. The suprachiasmatic nucleus has a circadian rhythm of its own: its firing, and therefore the intensity of the messages it sends onward, is also cyclical.⁴ The suprachiasmatic nucleus adjusts its own rhythm as new information from the eye reaches it, in doing so changing the timing of its messages. These changes in its activity transmit that information to the rest of the brain and body. In other words, the suprachiasmatic nucleus is our body’s “master clock”: it broadcasts the correct time to all the other cells in our body so they can synchronize their own clocks to each other and to the outside world. If you’re interested in learning more about our master clock, check out these previous PennNeuroKnowarticles about circadian rhythms.

When the light signals from the eye and the time on the “master clock” are very misaligned, it takes a while to bring your brain and body up to speed. Jetlag results from the suprachiasmatic nucleus gradually dragging all of our circadian processes back into sync with sunrise and sunset.  You’re probably not surprised to hear that messing with circadian rhythms like this messes with our mood. At a minimum, you’ve probably experienced how disruption to our sleep can cause irritability and mood swings.⁵ Stress-controlling hormones also have circadian rhythms, so we can become more anxious when our master clock is off.⁵

Interestingly, in an experimental setting, the circadian rhythms of patients whose mood symptoms tend to get worse at certain times of year shift more dramatically in response to light than those whose symptoms are steady year-round.² This suggests that the shifting sunlight during the winter is enough to disrupt these patient’s master clocks. Many such patients also find relief by using lamps that mimic natural sunlight, further supporting a direct role of sunlight in seasonal mood shifts.

In addition to altering circadian rhythms, sunlight impacts a chemical messenger called serotonin, which is heavily involved in mood regulation. The relationship between serotonin and mood is not straightforward, and genetic factors play a large role in how we respond to changes in the chemical.² However, we know that in people with Major Depressive Disorder or ahigh genetic risk of depression, losing serotonin worsens moods and increasing the strength of its message helps symptoms.⁵ This suggests that serotonin creates or stabilizes good moods.Exposure to sunlight strengthens serotonin’s message in a number of ways, such as increasing total levels of serotonin,² supercharging each individual message,^2,7 or making those messages last longer.^2,8 As you might expect, then, serotonin messaging strength naturally fluctuates throughout the year, including in the parts of the brain that control emotion and mood.^2,7,8The fluctuation in these messages there likely contributes to seasonal fluctuations in mood. Thus, serotonin provides a means by which sunlight can boost mood – and why we might get so glum in the dark of winter.

Atmospheric pressure

One of the major factors that control daily weather conditions is atmospheric pressure. This is the force that the air itself exerts against the ground as gravity pulls it earthwards.⁹  Generally speaking, high atmospheric pressure results in blue skies and fair weather. Meanwhile, lower pressure draws moisture upwards into the air, resulting in cloudy or stormy weather.⁹

The exact relationship between atmospheric pressure and mood is somewhat unclear. However, researchers have identified some factors. To start, atmospheric pressure is partially responsible for weather’s effects on serotonin.^10,11 In people who are already sensitive to weather, lowering atmospheric pressure can also activate the “fight-or-flight” arm of the sympathetic nervous system, which produces a state of stress and can therefore disrupt their mood.^10,11 It is also possible that the effects of atmospheric pressure could be due, at least in part, to pressure’s effects on sunlight. Because low atmospheric pressure brings cloudy weather, it will also decrease the amount of light reaching us – even on the longest summer days.

Have you ever encountered someone who can feel storms coming in their joints? Dramatic changes in pressure can worsen some people’s pain symptoms, which in turn can impact their mood. For example, low pressure fronts as well as rapid swings back to high pressure can trigger migraines in some patients.^12,13 Pain from osteoarthritis, a condition where the cushions between bones break down and cause joint pain, also worsens with lower pressure. Though the exact reason why pressure can increase joint pain is unclear, researchers currently think that lower pressure releases some of the force on blood vessels near facial nerves or the cushions between joints, allowing them to swell and trigger migraine or osteoarthritis pain.^14,15 These distracting, distressing pain symptoms can dampen mood. And since serotonin also has pain-relieving properties, reducing light-stimulated serotonin messages could contribute to pain attributed to atmospheric pressure changes.¹⁶

Conclusion

The relationship between weather and mood is so well-known that it’s built into the English language. Phrases such as “a ray of sunshine”, “smooth sailing”, and yes, “under the weather” show just how intuitive this is. But if the weather doesn’t faze you, you’re in good company too – there are large variations in how weather does or does not alter people’s mood. Even among mood disorder patients, between 15-47% are not affected by the season.² And a small subset of chronic patients experience worse pain with higher, not lower, atmospheric pressure.¹⁷ Genetic factors, such as those that underlie how light controls someone’s circadian rhythms, could be responsible for the variability in sensitivity to the cold darkness of winter. Cultural factors, such as the timing of holidays, might also play a role.³ Whether your mood changes with the weather or not, however, hopefully you’ll have a new appreciation for the next clear, cloudless day.

References

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6. Lambert GW, Reid C, Kaye DM, Jennings GL, Esler MD. Effect of sunlight and season on serotonin turnover in the brain. Lancet. 2002 Dec 7;360(9348):1840-2. doi: 10.1016/s0140-6736(02)11737-5. PMID: 12480364.
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12. Elcik C, Fuhrmann CM, Mercer AE, Davis RE. Relationship between air mass type and emergency department visits for migraine headache across the Triangle region of North Carolina. Int J Biometeorol. 2017 Dec;61(12):2245-2254. doi: 10.1007/s00484-017-1432-z. Epub 2017 Sep 13. PMID: 28900742.
13. Okuma H, Okuma Y, Kitagawa Y. Examination of fluctuations in atmospheric pressure related to migraine. Springerplus. 2015 Dec 18;4:790. doi: 10.1186/s40064-015-1592-4. PMID: 26702379; PMCID: PMC4684554.
14. McAlindon T, Formica M, Schmid CH, Fletcher J. Changes in barometric pressure and ambient temperature influence osteoarthritis pain. Am J Med. 2007 May;120(5):429-34. doi: 10.1016/j.amjmed.2006.07.036. PMID: 17466654.
15. Wang L, Xu Q, Chen Y, Zhu Z, Cao Y. Associations between weather conditions and osteoarthritis pain: a systematic review and meta-analysis. Ann Med. 2023 Dec;55(1):2196439. doi: 10.1080/07853890.2023.2196439. PMID: 37078741; PMCID: PMC10120534.
16. Pourhamzeh M, Moravej FG, Arabi M, Shahriari E, Mehrabi S, Ward R, Ahadi R, Joghataei MT. The Roles of Serotonin in Neuropsychiatric Disorders. Cell Mol Neurobiol. 2022 Aug;42(6):1671-1692. doi: 10.1007/s10571-021-01064-9. Epub 2021 Mar 2. PMID: 33651238.
17. Fagerlund AJ, Iversen M, Ekeland A, Moen CM, Aslaksen PM. Blame it on the weather? The association between pain in fibromyalgia, relative humidity, temperature and barometric pressure. PLoS One. 2019 May 10;14(5):e0216902. doi: 10.1371/journal.pone.0216902. PMID: 31075151; PMCID: PMC6510434.

Cover image from Pixabay user Istvàn Bogdan