Light the night? Monash research finds that some of us are hypersensitive to evening illumination

New research from Monash University has found human circadian clocks are “highly sensitive” to evening light, but that this sensitivity varies greatly among individuals.

The study found that exposure to typical dim indoor lighting before bedtime reduces the level of the sleep-promoting hormone melatonin, but there was more than a 50-fold difference in sensitivity in the people studied.

Led by Associate Professor Sean Cain, Dr Andrew Phillips and Dr Parisa Vidafar from Monash University’s Turner Institute for Brain and Mental Health, the study examined how evening lighting affects circadian rhythms. It’s the first project to systematically examine individual differences in light sensitivity in the human circadian system.

Participants were exposed to a range of light levels, including those experienced in the home, for five hours, starting four hours before bedtime, with researchers using a melatonin suppression measure of 50 per cent.  A light-sensitive hormone, melatonin signals the start of the “biological night”.

The study found that, generally, humans are highly sensitive to evening light. The 50 per cent melatonin suppression measure was reached at group level through exposure to just 30 lux, typical of evening home lighting, and the onset of apparent melatonin was delayed by 77 minutes.

“Electric lighting has fundamentally altered how the human circadian clock synchronises to the day/night cycle,” Associate Professor Cain said. “Evening light can make it more difficult to get to sleep because it suppresses melatonin, a sleep-promoting hormone.”

The researchers also found more than a 50-fold difference in sensitivity to light between the most and least- affected participants.

“We discovered that even when two healthy young individuals experience the same tightly controlled evening light environment, their melatonin suppression can differ vastly,” Dr Phillips said.

“For typical indoor light, we found some individuals respond as if exposed to bright, natural daytime light, whereas other individuals have almost no response.”

“Some individuals appear to be particularly prone to circadian rhythm disruption by lights we typically use in our homes, which could have massive knock-on effects on health,” Associate Professor Cain said.

The study, titled High sensitivity and inter-individual variability in the response of the human circadian system to evening light, was published by the Proceedings of the National Academy of Sciences of the United States of America on 27 May 2019.

Associate Professor Sean Cain