How seasons affect humans

How seasons affect humans

Life on this planet is adapted to the 24-hour (h) solar day. Over evolutionary time, the predictable daily cycles of light and dark have been internalized in the form of circadian rhythms. Before electric lights were introduced about a century ago, people were exposed to minimal light at night.

There is evidence of seasonal peaks in suicides, occurring more frequently in summer.

Birth rates tend to peak in spring and summer.

Seasonal affective disorder (SAD) usually begins in late autumn/fall or early winter and reduce/disappear in spring and summer. A study in 2001 found that humans secrete a hormone called melatonin for longer periods during winter nights than summer nights. Melatonin regulates sleep and those who don’t experience SAD don’t have differences in their melatonin levels in winter and summer. As much as 20% of the population is affected by Seasonal Affective Disorder.

Humans (and animals) have an internal clock and modern life is disrupting this natural occurrence. Our circadian rythmns are being affected negatively creating mental health issues.

Human ‘clocks’ don’t naturally work with the 24 hour rotation. Light resets our internal clock so that our bodies are in sync with the time of day. Again, modern life is messing with this natural pattern.

People with longer natural cycles tend to be night owls. Unfortunately, night owls face ‘sleep inertia’ after a late night and less rest that a ‘morning person’.

Humans rely primarily on light for their internal biological clock.

Late autumn and early winter are the best times of year for sperm health, as they are when men are most likely to have a higher sperm count.

Google sexual searches

More sexually related searches occur during the holidays and in early summer than at other times of the year. Dating terms are also more commonly searched as well.

A woman’s hands and feet tend to be colder in winter because their bodies are designed to stay warm at their core to protect their womb.

Cuffing season – the time people are most likely to couple up is the holiday season.

How modern life affects our circadian cycle

Mood disorders are often associated with disrupted circadian clock-controlled responses, such as sleep and cortisol secretion, whereas disruption of circadian rhythms via jet lag, night-shift work, or exposure to artificial light at night, can precipitate or exacerbate affective symptoms in susceptible individuals. Evidence suggests strong associations between circadian rhythms and mental health, but only recently have studies begun to discover the direct interactions between the circadian system and mood regulation.

The economic benefits of electric lighting enabled night shift work. By the end of the 20th century, technology provided people with additional sources of night-time light, including television, computer screens, e-readers, smartphones, and tablet computers.

Today, >80% of humans and 99% of those living in the US or Europe experience significant night-time light pollution. The artificial night sky glow is sufficiently bright that two-thirds of Europeans and nearly 80% of North Americans cannot see the Milky Way. Pervasiveness and intensity of nighttime light exposure is unprecedented in our history.

Light pollution isn’t good for our internal clocks

Exposure to light at night perturbs the circadian system because light is the major entraining cue used by the body to discriminate day from night. When exposure to light is mistimed or nearly constant, biological and behavioral rhythms can become desynchronized, leading to negative consequences for health.

Sleep disruption

Sleep disruption is a diagnostic criterion for major depression, bipolar disorder, post-traumatic stress disorder, generalized anxiety, and other mood disorders. Shift work disorder (SWD) is a circadian rhythm sleep disorder associated with working outside of the typical 800 to 1700 h shifts. Individuals with SWD report insomnia, difficulty falling asleep, and experiencing excessive sleepiness or micro-naps when it is important to be alert and productive. Affective responses associated with SWD include irritability, depression, and difficulty maintaining personal relationships. Shift work disorder can be provoked by night shifts, rotating shifts, afternoon shifts, or even early morning shifts. SWD is associated with chronic sleep deprivation and a persistent “sleep debt”.

Jet lag

In addition to light at night, circadian rhythms can be disrupted by another modern convenience, jet travel across time zones. Jet lag, also called jet lag disorder, is a transient sleep problem that arises when an individual travels across multiple time zone. Because circadian rhythms do not instantaneously reset, for several days they may remain more closely entrained to the original time zone than the current time zone; the lag in synchronizing these internal rhythms to the current photic (light) and non-photic (social interactions, timing of meals, etc.) cues results in disturbed sleep, daytime fatigue, hormone profiles, gastrointestinal issues, and changes in mood.

People who regularly cross multiple time zones, such as international flight crews, often report persistent jet lag symptoms; negative symptoms typically increase with the number of time zones crossed and are worse for travelers flying in an eastern direction than a western direction due to the necessity to advance the body’s clock.

Several studies have suggested that mood changes, especially dysphoric mood, are an important aspect of jet lag21. For example, a study of five men experiencing 7-h phase shifts (one westward shift and one eastward shift) over the course of a month reported that the eastward shift significantly disrupted sleep and elevated anxiety and depression scores.

Circadian rhythm disruption and major depressive disorder

Major depressive disorder (MDD) is characterized by alterations in mood, typically increased sadness and/or irritability that is accompanied by at least one of the following psychophysiological symptoms:

alterations in sleep, sexual desire, or appetite, inability to experience pleasure, slowing of speech or actions, crying, and suicidal thoughts.

Diagnosis of MDD requires these symptoms to persist for at least two weeks and interfere with normal daily functions. MDD affects vast numbers of people worldwide; the Global Burden of Disease Consortium examined the global incidence and prevalence for 328 diseases in 195 countries from 1990–2016, MDD ranked within the top ten leading causes of disease burden in all but four countries examined.

The incidence of MDD worldwide is on the rise; the number of people diagnosed with depression increased by ~18% from 2005 to 2015. Notably, rates of MDD correlate with modernization of society; which may reflect the increased circadian disruption (i.e., light at night, shift work, and jet lag) or the interaction between circadian disruption and other environmental factors experienced in modernized countries.

If possible, try to stick to a routin when it comes to your sleep pattern even if it means you will feel tired at times. sleeping too much is just as harmful as sleeping too little. Remember that our biology hasn’t developed as fast as technology has and as such, we need to honour our bodies and stick to traditional behaviours to protect out mental health.

Mandy X

 

 

Photo by Chris Lawton on Unsplash

 

 

References/Source:  https://www.nature.com/articles/s41398-020-0694-0#ethics