The term "circadian rhythm" describes the way bodily functions appear to follow a set pattern throughout the day as though working by an internal clock.
The internal clocks is disrupted for those affected by SAD. This leads a string of biochemical abnormalities and associated symptoms, like depression, tiredness, glycemic cravings and irritability. According to this theory, sunlight acts as a type of synchronizer of the circadian rhythms in humans, and reduced exposure to full-spectrum light shifts circadian rhythms out of phase.
The SAD sufferer's body may be producing hormones (such as cortisol) and neurotransmitters that promote wakefulness well into the early hours of the morning and then still be producing sleep inducing chemicals such as melatonin until midday.
This results in the unlucky sufferer not being able to sleep until past midnight and then not being able to drag themselves out of bed until late morning or later, which is a common situation.
It has been found that the timing of light exposure, rather than the amount of light exposure, may have the greatest influence over circadian rhythms, and hence SAD symptoms.
Understanding the major effect light exposure has on our physiology requires taking a deeper look at the hormone melatonin. The hormone melatonin's main function is to induce sleep, by traveling through the bloodstream and transmitting the sleep message to other body systems. As light levels decrease in the evening, the pineal gland begins producing melatonin.
In healthy individuals melatonin production peaks in the middle of the night during our deepest sleep. At dawn, sunlight shining into the eye triggers the pineal gland to switch off the production of melatonin, thus removing the desire to sleep. (This is one reason it's virtually impossible to sleep beyond daylight when sleeping outdoors in a tent during clear weather.) Melatonin travels to all parts of the body through the blood and has far-reaching effects.
During the hours of darkness and sleep, melatonin influences the secretion of hormones from the pituitary gland, often referred to as the "master gland" of the endocrine system. The pituitary then reduces hormone production from other important endocrine glands such as the thyroid and adrenals.
These glands produce vital hormones such as thyroid hormone, cortisol, and adrenaline, which control metabolism and motivate us to action during our waking hours. In effect, melatoni acts as a throttle governing the production of hormones that control many of the symptoms of SAD.
Research has determined this system is disrupted in people with SAD. When SAD patients were compared with healthy controls the SAD patients had consistently higher daytime melatonin levels during the winter months.
High daytime melatonin levels would be expected to produce the symptoms of excessive daytime sleepiness and the lack of motivation and desire to hibernate, that is seen in SAD sufferers.
Other research has shown that taking melatonin supplements, which are available over-the-counter in the US, effectively "phase-shifts" the disrupted circadian rhythms.
This means if you have SAD and you tend to get to sleep past midnight and wake well into the morning or midday, if you take melatonin at say 9-10pm to induce sleep, your sleep cycle will be shifted back to normal and you will be able to wake earlier in the morning.
Think you might have Seasonal Affective Disorder (SAD) ?
Common symptoms for SAD:
Common treatments for SAD mirror that of other types of depression; including talk therapy and anti-depressant medications. Light therapy, at specific times, using a full-spectrum light source to mimic the sun may also help.
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National Alliance on Mental Health: Seasonal Affective Disorder
Submit Your Theory
The serotonergic dysfunction theory of SAD states, based on research findings, that the receptors on brain cells that are stimulated by serotonin are not functioning correctly, resulting in abnormal neuroendocrine responses and the symptoms experienced in SAD.
Research now suggests that SAD has a genetic component. Simply put, low-sunlight is not an equal opportunity depressant. Some genetic groups are more affected by SAD than others. Specifically, the 5-HTTLPR gene has been found to be expressed differently in SAD patients.
We all know at least one person with Seasonal Affective Disorder (SAD -- aka the 'winter blues.' Check out 3 of the top theories behind the physiology of the SAD.
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