Photoperiodism is the reaction of plants and animals to the daily rhythm of illumination. The ability to measure the length of the day allows living things to determine the time of year and adapt to changing conditions. The specific mechanisms underlying the measurement of day length have not been fully understood. However, it is reliably known that changing the season affects body weight, immunity and the psyche of people.
How does the human body distinguish winter from summer?
Changes in the length of the day are weakly important for most animals, but they help to accurately determine the season of the year. There are 2 sources of information by which the human body distinguishes winter from summer:
- daylight hours
- the ratio between dark and light time of day.
The pineal gland (part of the brain) is responsible for determining the time of year in mammals. Removal of the pineal gland completely deprives a person of the ability to distinguish seasonal changes. Information about the light enters the brain through the eyes. Through the hypothalamus, the message is transmitted via the nerve cells to the pineal gland, which releases melatonin.
Melatonin is a sleep hormone that is formed in humans mainly at night. The maximum concentration of melatonin is observed at 2 a.m.
The duration of this nocturnal “peak” varies inversely with the length of the day in several animal species, including humans.
The results obtained in sheep and Siberian hamsters confirm the importance of melatonin in the regulation of "seasonal rhythms." Similar studies confirm that the concentration of melatonin is a “critical” physiological parameter that provides information about the season of the year.
Why do people gain weight fast in winter?
With the onset of winter, the human body significantly changes its own work in order to survive in harsh days. Changes in physiology are critical in both increasing survival and reducing long-term health effects.
To overcome the food shortage, the human body, which was found in ancient times, developed 2 adaptation mechanisms:
- weight gain;
- weight loss.
In the first case, a person used accumulated reserves of fat to make up for a lack of nutrients. In the second case, weight loss contributed to a decrease in energy requirements to maintain total body weight. Both energy saving strategies effectively saved people in the winter season.
How does immunity change in the winter season?
The transition from autumn to winter is manifested by changes in various aspects of the immune system. Short day lengths usually enhance many types of immune responses in the laboratory, although other specific protective functions are suppressed.
Enhanced immune defense in the short period of daylight counteracts the stressful effects of winter conditions, including reduced food availability and increased heat production requirements. This phenomenon of "seasonal plasticity" in the immune system persists and is observed in most people.
Most aspects of the acquired immune system, i.e. ability to recognize foreign molecules (viruses, bacteria), increase with decreasing daylight hours. However, most aspects of innate immunity, including inflammatory reactions, decrease in the winter.
Seasonal plasticity in the immune system raises an important conceptual question: "Why do immune responses always change depending on the season?"
In spring and early summer, small mammals invest heavily in the behavioral and physiological costs associated with reproduction. However, in wintertime, when successful breeding is usually impossible, "energy" investments are biased towards the immune system.
What mood disorders are more common in winter?
Excessive agitation or sadness is more common in the winter season.
Changes in food intake, decreased motivation (depression), as well as increased anxiety are observed not only in humans, but also in rodents.
Recent studies have identified several species of rodents that are literally “depressed” and anxious when winter sets in. In Siberian hamsters, depressive reactions intensify, and in lemmings, they are alarming.
In humans, changes are nonspecific: in studies, an increase in predominantly depressive moods was noted.
