If you find yourself feeling better the last couple of weeks, it could be because spring is just around the corner. During the winter months many people suffer with Seasonal Affective Disorder (SAD). They experience persistently low moods, irritability and lethargy.

The condition is thought to be linked to reduced daylight, which disrupts brain hormones and our internal body clocks. As the days get longer, spirits lift.

But mood can also be influenced by the intensity of light, not just the amount of light we are exposed to. This isn’t a seasonal phenomenon. It can affect us all year round.

Now new research has revealed for the first time that spending too much time in subdued light not only changes the way we feel, but can actually change the structure of the brain itself.

It’s a fascinating discovery.

Light Linked to Emotional Health

A number of studies reveal that mental health can be affected in some way by light. For instance, one group of female employees working in rooms with windows was compared to a similar group who spent their working day relying on artificial light.

Those exposed to natural light reported fewer symptoms of depression and better quality of sleep.¹

In another human study, a series of tests revealed that people feel emotions of any kind – positive or negative – with greater intensity under bright light. The initial reaction to a given stimulus appears to strengthen under such conditions.

But what is actually happening in the brain in response to light?

In their initial studies, neuroscientists from Michigan State University carried out experiments on Nile grass rats. Like humans, this breed of rat is diurnal and sleeps during the night.

The researchers found increased anxiety and depression among those kept in dim versus bright daylight conditions. They also found dysregulation in the hypothalamic pituitary adrenal (HPA) axis – the primary stress response system.³

In their most recent study, published in the March edition of Hippocampus, and funded by the National Institutes of Health, the Michigan scientists exposed the rodents to either dim or bright daytime conditions for four weeks. The wavelength of light spanned a range normally encountered by humans. Both groups had also been trained to navigate a maze as a way to enable the researchers to assess spatial memory.

Vital Brain Protein Plummets in Poor Lighting

The bright light rodents demonstrated marked improvement when they were retested in the maze. But the dim light rats lost nearly one third (30%) of their capacity in the hippocampus, a region of the brain vital for memory and learning. The dim light rats also failed to perform normally on the maze test.

These poor performers were returned to normal living conditions for a month before being exposed to another four weeks of light – this time, the bright kind. Their brain capacity and task performance made a full recovery.⁴

The researchers found that prolonged periods of time spent in ill-lit conditions caused a drop in brain-derived neurotrophic factor (BDNF), an important brain-growth protein that protects brain cells, encourages the creation of new ones, and promotes connections between neurons for better communication. BDNF is likewise vital in humans

Joel Soler, one of the authors, explained that in a dingy environment “…there are fewer connections being made. This results in diminished learning and memory performance that is dependent upon the hippocampus.

“In other words, dim lights are producing dimwits.”

Fellow study author Antonio “Tony” Nunez added, “When we exposed the rats to dim light, mimicking the cloudy days of Midwestern winters or typical indoor lighting, the animals showed impairments in spatial learning.

“This is similar to when people can’t find their way back to their cars in a busy parking lot after spending a few hours in a shopping mall or movie theater.”⁵