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A new study has revealed more about how exposure to dim light in the evening affects circadian health. The findings emphasise the need to optimise our artificial light exposure if we are to avoid shifting our biological clocks.

The research, led by Professor Stuart Peirson in the Sleep and Circadian Neuroscience Institute at the University of Oxford, shows that dim light in the evening delays circadian rhythms not only in the brain but also in organs throughout the body, including the heart, liver and adrenal gland. It also shows that these effects still occur in the absence of the blue-light sensitive circadian photoreceptor melanopsin.

In modern societies people are regularly exposed to artificial light, including the electric lighting in our homes as well as light-emitting electronic devices. Exposure to dim light in the evening has already been shown to increase our alertness before bedtime. It delays melatonin timing and sleep onset, and increases sleepiness the next morning.

In the new study, funded by the BBSRC and the Wellcome Trust, researchers used mice to investigate the mechanisms underlying behavioural responses in more detail. Humans and mice have similar circadian responses, even though mice are nocturnal. The researchers exposed the mice to four hours of light that was ten times lower than the light they experienced during the day, every evening for two weeks. This is comparable to the light levels experienced by humans under electric lighting conditions.

They discovered that in mice, exposure to dim light in the evening delays activity rhythms, sleep and body temperature. In addition, the normal rhythm of learning and memory performance is reversed, which may be related to changes in sleep/wake timing.

These findings will allow the researchers to work out how change the intensity and wavelength (colour) of artificial light during the day and in the evening to help avoid these disruptive effects. They will also be able to investigate whether long-term exposure to dim light in the evening has adverse health consequences.