What do ice cream sales, outdoor temperatures, and violent crimes have in common? They all increase in the summer. There are a few reasons that violent crimes could increase in the summertime. One being that people get bored during the summer and therefore find things, such as crime, to occupy their time. Another is that as temperatures increase, people become more irritable and aggressive. Hood and Amir (2018) would argue, however, that anger and aggression are associated with an infradian cycle correlated with the seasons.

Your body’s internal clock, the circadian and infradian rhythms, are an integral part to your daily life. Predictable cycles, or rhythms, in behavior are expressed on several different time scales such as circadian (circa diem, or approximately 24-hour rhythms) and infradian (exceeding 24-hours, such as a monthly or a seasonal cycle). The most well-known circadian rhythm is the “sleep-wake” cycle and examples of infradian cycles are menstrual cycles, seasonal breeding, and migration behaviors.. However, in addition to regulating your sleep cycle, the circadian timekeeping system also influences anger and aggression. Conversely, prolonged anger can also influence and disrupt your circadian rhythm. The suprachiasmatic nucleus, the SCN, in the hypothalamus contains the master circadian clock, and exposure to daylight is what regulates this cycle. Other environmental factors, such as social interactions and eating food, also influence both the circadian and infradian rhythms of humans and other species (interestingly primates are both the only living things that have circadian and infradian cycles, many plants do as well).

Studies of other animal species have shown the strongest evidence for seasonal rhythms of anger and aggression. Many different species of mammals, birds, reptiles, fish, and insects show very predictable shifts and peaks in their aggressive behaviors throughout the year. These aggressive behaviors are also linked with other seasonal behaviors such as mating and territory defending. In humans, the environmental factors that influence infradian rhythms of aggression are temperature and average hours of daylight.

However, some types of aggression are correlated with seasonal changes while others are not. For example, crimes that involve personal attacks, such as aggravated assault and domestic violence, are more likely to occur during the summer months than in the winter moths. In contrast, crimes that don’t involve personal attacks, like robberies, are not correlated with the seasons at all (Schreiber et al.,1997). Murder rates also do not show seasonal trends, whereas suicides using both violent (i.e. firearms, hanging, and drowning) and non-violent (i.e. overdose) methods increase during the spring.

An individual’s circadian rhythm can be used to predict their aggressive behaviors using whether they are a morning or a night person. Specifically, young and middle-aged adults that self-identify as night owls tend to score higher on tests of impulsivity, anger, and irritability. In addition, parents and teachers of children and teens that are night owls report that they lie, scream, and swear more than children and teens that are morning people.

In many older adults that have Dementia of the Alzheimer’s type, effects of their circadian rhythm on aggressive behaviors are also present. In the late afternoon and into the night, many of these patients experience “sundowning” (reviewed in Bachman and Rabins, 2006). Sundowning is characterized by confusion, anxiety, aggression, ignoring instructions, and pacing or wandering. Although related to a specific time of day, there is actually evidence that sundowning is actually a phenomenon related to the caregivers, and not the patient themselves. It is theorized that caregivers are more likely to report behaviors associated with sundowning in the afternoon/evening, rather than the patients actually exhibiting more aggressive and confused behaviors.

Overall, our internal circadian and infradian rhythms are heavily influenced by changes in the weather and shifts in daylight. These rhythms affect more than just sleep cycles in humans. A multitude of species have been observed to have these cycles, and these cycles can predict such things as violent crimes suicide in adults, and aggressive behaviors in children and teens, and (potentially) confusion and aggression in the elderly.

Sources:

Bachman, D., & Rabins, P. (2006). “Sundowning” and Other Temporally Associated Agitation States in Dementia Patients. Annual Review of Medicine, 57(1), 499-511. doi:10.1146/annurev.med.57.071604.141451

Hood, S., & Amir, S. (2018). Biological Clocks and Rhythms of Anger and Aggression. Frontiers in Behavioral Neuroscience, 12. doi:10.3389/fnbeh.2018.00004

Schreiber, G., Avissar, S., Tzahor, Z., Barak-Glantz, I., & Grisaru, N. (1997). Photoperiodicity and annual rhythms of wars and violent crimes. Medical Hypotheses,48(1), 89-96. doi:10.1016/s0306-9877(97)90029-3