Why Weather Affects Mood

Even as many of us are trapped indoors due to the pandemic, our moods rise on bright days and plumb the depths after days of rain. Such transitory mood swings are not well understood but animal behavior offers useful insights.

Adaptive Inactivity

Low moods are demotivating. We feel lethargic, or tired, and are less likely to exert ourselves in new, or productive, activities. While we may feel listless, dispirited, and unhappy, this could boost the biological currencies of survival and reproduction.

We wake up when it gets bright in the morning so that our activities follow a rough circadian rhythm, as do internal physiological systems from digestion to immune function. For example, we eat less often in the nighttime because insulin production slows and less sugar is withdrawn from the bloodstream, which blunts hunger.

Knowledge about the restorative value of sleep is expanding by leaps and bounds, from its effects on immune function to memory and flushing the brain of impurities (1). Feeling tired encourages us to lie down and sleep each night.

The same sort of circadian rhythm is manifested in most other vertebrates although some, like owls, become active at night. Cycles of rest and activity are surprisingly flexible and shift workers are forced to reverse the usual pattern that can impose health costs. Similarly, species that are normally diurnal can shift to becoming nocturnal due to human influence, as is true of coyotes, boars, elephants, and tigers, who avoid unwanted human attention by becoming active at night.

There are strong seasonal patterns for some species that rest more in winter. Some go to the extreme of hibernating when they sleep in a warm den through much of the cold season. In spring, longer days rouse hibernators from their slumber and motivate some species to embark on their seasonal migration.

Light and Reproduction

These effects are illustrated by the way that day length alters the hormonal status of seasonal breeders like birds. With increased day length, migratory species become increasingly restless and head off in the direction of their seasonal migration.

In spring, as day length increases, male birds experience a surge in testosterone, fight over territories, and defend them with the territorial song. Singing is itself related to testosterone because song controlling structures of the brain wax and wane with the breeding system.

Humans are not seasonal breeders but our brains also have complex responses to day length. These are highlighted by the phenomenon of wintertime depression or seasonal affective disorder, that occurs in highly seasonal places distant from the equator where day length in winter is very short and triggers severe depression in vulnerable individuals.

Although we are not a seasonal species, our reproductive systems are affected by the length of days. Men have an annual cycle in circulating testosterone that peaks in autumn.

The Seasonal Shift

Most of us tolerate the short days of winter, although being confined at home due to extreme cold interferes with our customary activities and thereby lowers mood.

When temperatures warm up in summer, we spend more time outdoors and are more physically active whether this involves sports activities, exercise, or outdoor hobbies like gardening. Most people prefer milder temperatures and express greater feelings of optimism and joy.

One side effect of being outdoors longer is that violent crime rates go up. The fact that most civil unrest occurs in midsummer used to be attributed to extreme heat increasing irritability and aggression but there is no evidence of this. If anything, extreme heat makes us unwilling to move around, much less attack anyone.

The demotivating effect of extreme summer heat is similar to that of extreme winter cold. Both are stressful tending to increase anxiety and lower mood.

Stress and Precautionary Behavior

Extreme winter cold and extreme summer heat crimp everyday activities in similar ways. In cold climates, there is a temptation to stay indoors more and to get less exercise. When a person does brave the elements, they must spend time putting on extra winter gear that has to be removed on their return.

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This is a time-consuming annoyance that we do not have when the weather is mild.

When the weather is hot, we have the trouble of putting on protective cream to prevent sunburn and wearing sun hats. Humid conditions are highly uncomfortable and demotivating. This means that we spend more time in air-conditioned buildings and vehicles and spend less time outdoors.

Whether hot or cold extreme conditions can be highly unpleasant and demotivating. We must also spend a lot of money to control the environment in our homes. All of these aspects of extreme weather make them potentially stressful, anxiety-provoking, and depressing.

Despite this, there is surprisingly little evidence that climate has any reliable impact on mood or mental health. There is a very simple explanation for this which is that we are good at adapting to the particular conditions in which we find ourselves.

Residents of Minnesota are accustomed to long, hard winters and many enjoy spending time outside pursuing winter activities such as skating and ice fishing. They are accustomed to the extreme cold and take it in their stride. There is also some physiological adaptation with greater bodily heat production after prolonged cold exposure.

So we encounter a strange paradox in which harsh weather is ostensibly stressful but has little obvious impact on mood because we are good at adapting to varied environmental conditions. This trait may be what allowed our ancestors to occupy Europe and Eurasia when they were in the grip of an ice age.