A new understanding of how antidepressants work: enhancing the effect of light on the body clock





1. Please give a summary of your research

Light has powerful effects on the brain and body, including the synchronisation of our internal clocks to the external environment, and increasing mood and alertness. We know that virtually all tissues of the human body contain 24-hour clocks, and that their alignment promotes optimal health, while their misalignment leads to disease.

In my PhD, I showed that patients with depression have abnormal light sensitivity, resulting in the disruption of internal clocks. This leads to poor sleep and mood, and may explain the development of depression for many patients. Further, I discovered that the most common antidepressant medications (SSRIs) profoundly increase the effect of light on the clock, and that this effect partly determines whether the medication is effective in an individual. An increase in sensitivity will likely cause “jetlag-like” symptoms in some patients, exacerbating sleep and mood problems, and preventing recovery. Alternatively, it may explain how antidepressants are effective in others – by boosting the ability of the internal clocks to synchronise with the environment, and enhancing the positive effect of light on mood and alertness during the day.

These findings represent a previously unknown mechanism of action for antidepressant medications, which could fundamentally change the way these medications are prescribed. Further, through simple prescription guides we may be able to dramatically increase the number of people who benefit from antidepressants. Given ~10% of Australians are currently taking an antidepressant, my research may have profound implications for wellbeing in our country. 

2. Please include any additional details you would like to share

Our publications relating to this work can be found below:

McGlashan, E. M., Coleman, M. Y., Vidafar, P., Phillips, A. J. K. & Cain, S. W., (2019). Decreased sensitivity of the circadian system to light in current, but not remitted depression. Journal of Affective Disorders, 256, 386-392.

McGlashan, E. M., Nandam, L. S., Vidafar, P., Mansfield, D. R., Rajaratnam, S. M. W., & Cain, S. W. (2018). The SSRI citalopram increases the sensitivity of the human circadian system to light in an acute dose. Psychopharmacology, 235(11), 3201-3209.

McGlashan, E. M., Drummond, S. P. A., & Cain, S. W. (2018) Evening types demonstrate reduced SSRI treatment efficacy. Chronobiology International, 35(8), 1175-1178.


The work has also been featured in a number of news articles, some of which can be found below:







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Dr McGlashan is a research fellow in the Sleep and Circadian Medicine Laboratory, at the Turner Institute for Brain and Mental Health - Monash University.

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