'Tis The Season: Circadian Rhythms
- Tuesday, 23 December 2008
The holidays are here, and that means it’s time for another yearly occurrence, albeit one that comes without tinsel and Santa: It’s the Winter Solstice, the shortest day and longest night of the year.
Although they often can’t explain why, many people complain of feeling sluggish or suffering from a bad mood during this typically cheery time of year. But, biologically, these feelings make perfect sense when they occur around the Winter Solstice—which typically falls between Dec. 20 and 23 in the Northern Hemisphere, depending on the year.
Our eyes actually suffer from a lack of daylight during this time of year, a reality which can have significant effects on our mood as well as the quality and amount of sleep we’re getting.
The mental health benefits of daylight are not unknown to the medical field. Physicians and psychiatrists alike have long advised their patients to seek adequate light in order to promote physical and mental health.
In recent years, the benefits of exposure to bright light—namely, improved mood, sleep, and daytime energy levels—have become increasingly supported by evidenced-based studies in the scientific literature. 
To fully understand the benefits of exposure to light, it is important to understand some key terminology surrounding the hormones and other neurochemicals involved in sleep and mood regulation.
Melatonin is a light-sensitive and naturally occurring hormone that is one of the key players in regulating your body’s circadian rhythm, the 24-hour cycle of physiological and behavioral processes of human beings. When melatonin is on the rise in your blood, it is responsible for inducing sleepiness and lowering your body temperature. This generally takes place at night, when one would be preparing for bed anyway.
Often times, however, due to individual variance in circadian rhythms, or lack of light exposure, melatonin can be abnormally high at the start of the day. This can lead to difficulty waking up and can promote a general feeling of grogginess throughout the morning, something many of us have often guiltily remedied with a large cup of coffee!
By exposing yourself to bright light in the morning, however, melatonin levels in the blood are rapidly reduced, and that bleary feeling is zapped.
Therapeutic light exposure is also directly energizing and has been proposed to stimulate the brain’s serotonin system.  Serotonin plays an important role in the brain and throughout the body as it modulates many biological processes, including mood and sleep.
By exposing yourself to bright light in the morning, you can manipulate the levels of melatonin and serotonin circulating in your blood.
As we head into the depths of winter and our opportunities for morning sun are fewer and further between, it is still important—and effective—to get outside for some daylight exposure. This is additionally helpful for general physical fitness. In the event that a walk outside isn’t enough for your mood, and you genuinely feel that the change in season is affecting you negatively, I would recommend a consultation with your physician. They may recommend a visit with a psychiatrist, or artificial bright light therapy delivered via a light box, which can be individually tailored to you based on your symptoms.
In the meantime, however, continue to make every effort to get outside during the day. The sweet smell of pine trees and eggnog alike await you upon your return.
- Eastman et al. Bright Light Treatment of Winter Depression. Arch Gen Psychiatry 1998; 55:883-889.
- Terman et al. Light Therapy for Seasonal and Nonseasonal Depresion: Efficacy, Protocol, Safety and Side Effects. CNS Spectr. 2005;10(8):647-663.