"See, Winter comes, to rule the varied year, Sullen and sad, with all his rising train; Vapors, and Clouds, and Storms." ~ James Thomson.
Well past the solstice, winter lingers with its short days and long dark nights in which to ponder how we are attuned to life on this planet of 24 hour solar days and seasonal variations. Over evolutionary time, the periodic cycles of light and dark became internalized as circadian (circa = about, dies = day) rhythms that synchronize living organisms to the external environment, and allow them to anticipate events such as availability of food, light and mates. Organisms such as plants, bacteria, fungi, fish, amphibians, reptiles, insects, and mammals, including humans, have internal circadian rhythms of approximately 24 hours, and are reset daily to precisely 24 hours through exposure to light–dark cues. In the absence of external signals - say, you live in a cave or bunker - circadian rhythms persist, but start to run slightly longer than 24 hours. Thus, light is the main cue to maintain synchrony with the environment.
Timing of many physiological events is coordinated by our brain’s circadian master clock - the suprachiasmatic nucleus (SCN) of the hypothalamus. SCN is driven by light falling on specialized retinal cells called intrinsically photosensitive retinal ganglion cells (ipRGCs) which express a photo-pigment that makes them exquisitely light sensitive. Blue light most strongly stimulates the photo-pigment whereas red light has minimal effect. Interestingly, the characteristics of sunlight change across the day such that it contains more blue wavelengths during the day and shifts to red wavelengths toward sunset. Thus the ipRGCs are adapted to the natural solar cycle, can discriminate daylight from evening, and communicate that information to SCN, as well as other targets in the brain.
Two ways circadian rhythms in humans manifest are the daily cycles of melatonin and cortisol production. Time of day information is sent from SCN to the pineal gland to regulate production of melatonin which occurs primarily at night. Secretion of melatonin is tightly controlled by light and even modest light exposure at night suppresses it, and can disrupt sleep. Additionally, our circadian system regulates cortisol secretion from the adrenal glands, such that concentrations peak in the morning just before awakening and decrease throughout the day. Cortisol affects many functions including physiological and behavioral responses to stress. Light exposure at night affects cortisol secretion, and may promote conditions like cardiovascular and metabolic disorders, and major depressive disorder.
Our sensitivity to light and its effect on circadian rhythms means the shorter days of winter can alter our physiology, behavior and mood. With less light, our brain sends signals to conserve energy, slowing metabolism and increasing hunger. Melatonin may be overproduced in longer dark or dim light conditions, making us feel sleepy and sluggish. Decreased light signals to hypothalamus and other brain regions may affect attention and cognitive function. And, effects mediated through the pineal gland as well as by direct connections to mood-related areas like amygdala trigger emotional shifts and depression.
While some degree of feeling blue is a normal response to winter for a lot of us, about 2-5% of people in the US experience pronounced difficulty from fall through spring each year, and may be diagnosed with Seasonal Affective Disorder. Symptoms include low energy, sleep pattern disruption, trouble focusing, social withdrawal, overeating, and depressed mood. Strategies such as light exposure, vitamin D, exercise, and healthy eating can help, but many will need to work with a therapist or even take medication just to get through the dark winter.
Post by: Nadia Fike
Read more: 1. Rhythms of Life: The Biological Clocks That Control the Daily Lives of Every Living Thing. Kreitzman, L. and Foster, R. Yale Univ. Press, 2004. 2. Intrinsically Photosensitive Retinal Ganglion cells of the Human Retina. Mure, L.S., Front. Neurol., 25 Mar 2021, Sec Neuro-Ophthalmology,Vol 12. https://doi.org/10.3389/fneur.2021.636330
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