Why Is Depression Increasing in the US?
A Japanese Study Suggests an Answer
Elizabeth Saenger, PhD
Depression has been increasing significantly in the US lately. For example, a study from the Mailman School of Public Health at Columbia University found that in a recent ten-year period, depression rose from 6.6% to 7.3% in the population as a whole. This meant that the number of Americans with depression increased from 21.6 million to 23.9 million.
The young (aged 7 to 12 years old) had the greatest increase, but other groups also showed significant increases, such as:
- the elderly
- people with the lowest and highest incomes
- those with the highest levels of education.
Increases in depression are a serious problem. This mood disorder can be disabling for those who have it. Further, not only are the symptoms a problem in and of themselves, but depression is associated with risks of dementia, cardiovascular disease, and early death. In addition, depressed individuals often tax family members, friends, employers, and society at large.
Research Sparks a Study
Recently Kenji Obayashi, PhD, and his colleagues focused on the connection between depression, or depression-like behavior, and exposure to artificial light at night. This connection appears in:
- hamsters who were exposed to a low level of light at night for four weeks
- night shift workers
- elderly people who were exposed to artificial night at night.
These scientists were especially interested in the association between depression and evening light exposure in the elderly. Their earlier research on that topic had compared such light exposure in two groups: people who were depressed, and people who were not. It concluded that the depressed people (compared with those who were not) had been exposed to light at night more often, and more often for longer periods of greater intensity.
However, the study Dr. Obayashi and his colleagues conducted was cross-sectional (a snapshot in time) vs longitudinal (observations of how people changed over time). The researchers wanted to do a longitudinal study which would let them determine whether light at night was a risk factor for depression in elderly people living at home.
A Risk Factor for Depression
To test this idea, the researchers recently investigated whether exposure to light at night in the bedroom was an independent risk factor for depression in the elderly. Their investigation had several strengths, including:
- a longitudinal design
- a large sample size (863 participants)
- a precise method of measuring bedroom light while subjects were actually sleeping, thus approximating how much light hit the retina, where light begins the process of carrying a message to the inner clock (the suprachiasmatic nucleus in the hypothalamus).
The subjects were exposed to light at very low levels, made still lower by the fact that subjects usually had their eyes closed. Nevertheless, this miniscule amount of illumination at night increased the occurrence of subsequent depression.
Further, exposure to brighter light, even within those low levels, was associated with a greater risk of depression. In short, it seemed that if the brightness of the light exceeded a threshold, albeit a low threshold, it had negative consequences.
The Next Question
Are children and adolescents even more vulnerable than the elderly to light at night? After all, as we age, our lenses yellow, and our pupils become smaller, and less flexible. Therefore, less light strikes our retinas. That means that at age
- 45, we receive only half as much light in our circadian systems as we did in our youth
- 75, we receive only a third of what we received when we were 45.
Perhaps the sensitivity to bright light that youth have is associated with the finding, noted above, that children aged 7 to 12 experienced the greatest increases in depression. This raises the question of whether exposure to light at night might cause depression in children.
Dr. Obayashi and colleagues will be investigating light at night as a risk factor in depression for children. Stay tuned!
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