Sensitivity to light is vital for good health, because our brain uses light levels to set our internal clock. If your brain thinks it is darker than it is, it will tell you it is night time – making you sleepy and lethargic. These are common symptoms of depression. “These people cannot move their clock to match the change in the day,” says Professor Ian Hickie, a co-director at the University of Sydney’s Brain and Mind Centre, who was not involved in the study. “It’s better thought of as being terminally jetlagged rather than terminally depressed. You feel sick, you cannot experience joy, you cannot sleep." Lead scientist Associate Professor Sean Cain, a psychology researcher at Monash University, believes antidepressants make people feel better by increasing their sensitivity to light to a normal level. That allows people's body clocks to properly sync with the day. They feel much better – just as people do when they finally shake off their jetlag from an international flight.

Sunlight is also proven to have a powerful effect on mood. By boosting light sensitivity, sunlight would be even more powerful, Professor Cain believes. “It’s like light therapy in a pill,” he says. The study may also answer a major question about antidepressants: why they seem to work better for early risers than night owls. Light from screens at night can upset body clocks. Credit:Michelle Mossop People who work nightshifts, or watch TV or use their phone in bed are exposing themselves to bright light late at night, which can mess with their body clock by tricking their brain into thinking it is day.

If they are using an antidepressant, their heightened light sensitivity might magnify the effect. If the results hold up to further study, Professor Cain believes doctors could one day advise antidepressant users to avoid bright light at night. “We think this discovery could help millions,” Professor Cain says. “Your behaviour could strongly influence whether these drugs work or not.” Loading Replay Replay video Play video Play video The study gave a single dose of citalopram (sold as Cipramil or Celexa) to 12 people who did not experience depression. Each person’s light sensitivity – measured by tracking their melatonin changes when staring at a lamp – increased by an average of 50 per cent after they took the pill.

“You never see this”, says Professor Cain. “We gave them a single dose – and it had a massive effect.” However, the resulting paper, published in August in Psychopharmacology, has significant limitations. The sample size was small, and it only shows a link between citalopram and light sensitivity, not light sensitivity and depression. It is also not yet established how long-term antidepressant use would affect light sensitivity. But Professor Hickie, a former national mental health commissioner, said the research should be “taken very seriously”. And Dr Andrew Phillips, a circadian rhythms researcher at the Monash Institute of Cognitive and Clinical Neurosciences, called the study “potentially a landmark”.