We all feel a little ‘blue’ over the winter period. With the days getting shorter and cold setting in, it’s no wonder we find it harder to be our usual ‘perky’ selves. But for some people, this feeling is far more extreme. For those with seasonal affective disorder (or SAD for short), the winter months each year mean a period of significant depression, fatigue and a loss of interest in the activities they would usually enjoy.
Despite some ongoing cynicism, SAD is classified as a medical condition by the American Psychiatric Association – though the individual must already be diagnosed with a major depressive or bipolar disorder and should have experienced SAD symptoms for at least two consecutive years [1,2]. What is interesting about SAD, however, is that in contrast to the typical symptoms of depression, individuals with SAD often experience hypersomnia (an increased desire to sleep) rather than insomnia and a heightened rather than reduced appetite, resulting in weight gain [1,2].
So far there is no consensus as such on the causes of SAD but it is generally agreed that seasonal changes, primarily shorter light periods and lower levels of environmental light available, play a significant part. This is supported by the use of light therapy to successfully treat up to 70% of individuals with SAD [1]. But how does a lack of light translate into SAD?
One of the major theories relating light to SAD involves our circadian system, known more anecdotally as our ‘biological clock’. This system controls our daily (and seasonal) cycle, dictating when we feel alert and sleepy, when we get hungry, and being responsible for the onset of hibernation in certain (some may argue more sensible) animals. Our circadian system responds to environmental cues, principally light, using these signals to sync our body clocks to the outside world through the release of chemicals which indicate when it is most appropriate to eat, be active or sleep [3]. In individuals with SAD, this delicate and complex system is believed to be disrupted, leading these people to become ‘out of phase’ with their environment, upsetting their sleep and eating patterns and causing them to become depressed.
Much of the research behind the circadian theory to date has focused on melatonin, one of the key components of our circadian systems and the chemical responsible for making us sleepy. Under normal circumstances, our bodies release melatonin at night and stop in the morning in response to light. This allows us to sleep when it is most appropriate. In some people with SAD, however, this cycle appears to be out of sync, with melatonin being released either earlier or later than usual [4]. By normalising this release pattern using either light therapy or the administration of melatonin itself, it may be possible to relieve the symptoms of SAD, and a number of studies have been carried out which support this hypothesis [3,4].
A second theory linking light and SAD looks at the eyes, or more specifically the retinas, of people with SAD, suggesting a lower sensitivity of these structures to light. Under normal circumstances, our retinas increase their sensitivity in response to low light conditions, i.e. dark winter days. In individuals with SAD, however, this may not happen [2]. Studies designed to test retinal function by measuring the electrical response of the retina to light have found that the retinas of some people with SAD are less responsive to light in the winter compared to the summer and in relation to healthy controls, lending support to this theory [3,4].
The third and final theory we’ll discuss in this article involves a family of signalling chemicals found in our brains, known as monoamine neurotransmitters. Members of this family, namely serotonin and noradrenaline, are known to affect our mood, eating and sleeping habits, making it logical to suggest they may be involved in the biological basis of SAD [1]. They also appear to respond to light availability and time of year. Our serotonin levels, for instance, are higher in summer than winter [3,4]. In some people with SAD, levels of serotonin and noradrenaline seem to be lower than in healthy controls. Increasing these back to the norm using either light therapy or drugs which promote serotonin or norepinephrine production has been shown to improve mood in these individuals and relieve their SAD symptoms [3,4].
As is so often the case with medical conditions, particularly those involving our mental health, our understanding of the causes behind SAD is still somewhat hazy. However, irregular responses to low-light environments found in SAD sufferers, whether it be through abnormal melatonin production disturbing their circadian systems, less sensitive retinas, or atypical levels of neurotransmitters, does seem to be a major factor. Both time and further investigation are needed to understand fully the biological causes of SAD and improve therapy options. Nevertheless, for those suffering with this condition, rest assured there is light at the end of the tunnel (do excuse the pun).
Post by: Megan Barrett
References
Gupta A, Sharma P, Garg V, et al. Role of serotonin in seasonal affective disorder. Eur Med Pharmacol Sci 2013; 17: 49–55.
Roecklein K & Wong P. Seasonal affective disorder. In: Gellman M & Turner J (eds.). Encyclopedia of behavioural medicine. New York: Springer; 2013. p1722–4.
Danilenko K & Levitan R. Seasonal affective disorder. In: Schlaepfer T & Nemeroff (eds.). Handbook of Clinical Neurology, Vol. 106: Neurobiology of psychiatric disorders. 3rd series. Amsterdam: Elsevier; 2012. p279–90.
Rohan K, Roechlein K, Haaga K, et al. Biological and psychological mechanisms of seasonal affective disorder: a review and integration. Curr Psychiatry Rev 2009; 5: 37–47.
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