Seasonal Affective Disorder and Cognitive Functioning

Question:

“In a previous Q&A, you discussed vitamin D deficiency in depression. Could you further elaborate on seasonal affective disorder and cognitive functioning?”

Jon W. Draud, MD, MS:   This is a great question. The prevalence of seasonal affective disorder (SAD) seems to range from 1.4% in Florida to 9.7% in New Hampshire. ¹  SAD was first described by Norman E. Rosenthal and colleagues at the National Institute of Mental Health in 1984. ^2-4  Treatments range from light therapy and ionized-air administration to use of melatonin, as well as cognitive therapy and antidepressant pharmacotherapy. There is a great deal written about SAD, but I’d like to concentrate on the role of sunlight exposure and cognition.

A recent study examined the effects of sunlight exposure on cognitive function among patients with depression and patients without depression. ⁵  The rationale stems from the fact that suprachiasmatic nuclei (SCN) seem to link sunlight and mood as evidenced by serotonin and melatonin and their roles in depression. Cognitive functioning is involved in these same pathways and seems to be affected by sunlight. ^6-8  The study examined 16,800 participants and posed the question of whether a close relationship existed with sunlight exposure and cognition in depressed versus non-depressed patients. The participants were 45 years and older and evenly distributed among 48 states, with 45% being male and 55% being female. A six-item cognitive questionnaire was used to measure cognition before and after sunlight exposure. The cognitive screener was divided into two parts: short-term recall and temporal orientation.

Participants with depression who had lower sunlight exposure had a statistically significantly higher level of cognitive impairments. Among non-depressed participants, sunlight exposure did not seem to affect cognition. The exposure was either a single day exposure or that given daily for a period of two weeks. There was, in fact, a dose-response relationship found between cognitive function and sunlight exposure (odds ratio=2.58; 95% confidence interval=1.43-6.69) such that lower levels of sunlight are linked to impaired cognition. This adds to the evidence that shows that lifestyle and environment have a profound effect on patients with SAD.

Data suggest that violent homicides, suicides, and aggression peak in late spring and are linked to climatic variables and sunlight. ⁹  It is now well-established that SAD depressive episodes are linked with the shorter daylight periods seen in winter. ¹⁰  This new evidence that sunlight exposure is linked with cognition in patients with depression gives further credence to the idea that the physiology giving rise to seasonal depression may also be involved in sunlight’s effects on cognition. Leonard and Myint have even linked a lack of sunlight to altered serotonin levels, neurodegeneration, depression, cognitive changes, and even dementia. ¹¹

Environmental illumination has been shown to be important in both seasonal and non-seasonal depressions. ^10,12,13  These theories linking seasonal cycles, depression, and cognition are based on circadian rhythmicity and the SCN. ^3,14  The SCN are modulated by body temperature and physical activity, but specifically via light received by retinal sensors at wavelengths approximating 477 nanometers, which is close to “natural sunlight.” ¹⁴  The SCN regulate numerous processes including the immune system, hormone systems, blood pressure, digestion, and sleep cycle and body temperature. Walker and Stickgold ¹⁵  have linked dysfunctional circadian rhythms with cognitive deficits, and Miller et al ¹⁰  have shown that one of the SCN regulatory functions is to inhibit the pineal gland from converting serotonin to melatonin in the presence of daylight. Serotonin and melatonin have been implicated in many mental and cognitive disorders, including Alzheimer’s disease, various sleep disorders, and Parkinson’s disease. ^16,17  Abnormalities in these systems have been shown to exist in bipolar disorder, ⁸  schizophrenia, ¹⁸  SAD, ^16,19  and in those with no psychiatric illness. ²⁰

Furthermore, light seems to affect blood flow to brain. Dani et al ²¹  have shown that cerebral flow improves in infants after phototherapy, as well as patients with SAD, ²²  which of course is linked with improved cognition and memory.

A unique and important point about the Kent et al ⁵  study mentioned earlier is that NASA satellite data were used to obtain natural sunlight exposure in the participants, whereas most existing studies are with artificial light sources.

Finally, this new finding that weather may affect cognition, as well as mood, has important clinical implications. It suggests that sunlight exposure has an independent relationship with cognition and depression, and that light therapy used in SAD may improve cognition, as well as mood. Hopefully future studies will continue to shed light,  every  pun intended, on these interesting clinical issues for us and our patients.

—Jon W. Draud, MD, MS

References

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