Coloured lights make mice sleepy – and stressed

Tossing and turning at night? The glow of your phone may be tinkering with stress hormones, a new study suggests. Amy Middleton reports.

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Like to read your smartphone or tablet before bed? Have a digital alarm clock that floods your bedroom with light through the night? The wavelengths that bathe you before and as you sleep may stress you out too, a new study in mice suggests.

Mice are nocturnal animals, and when they are shown coloured light during the night – usually their awake hours – they tend to fall asleep. But at the same time, stress hormones such as corticosterone, which is produced by the adrenal gland, get a bit of a boost too.

So researchers at Oxford and Bristol Universities in the UKs set about teasing apart these two contradictory reactions.

“We wanted to understand how these two effects were related and how they were linked to a blue light-sensitive pigment called melanopsin, known to play a key role in setting our body clock," explains Stuart Peirson, a neuroscientist at Oxford and co-author on the study.

They published their work in PLOS Biology.

Melanopsin is a light-sensitive protein found in retinas of humans and mice. Its stimulation by different wavelengths, or colours, of light helps control our circadian rhythms – that is, the internal awareness that regulates sleep and wakefulness.

The researchers expected blue light would send mice to sleep the fastest, given that melanopsin is most sensitive to blue wavelengths.

But their findings suggested otherwise. Green light induced sleep the fastest, sending mice into slumber in just one to three minutes.

Violet light was slightly less powerful, taking five to 10 minutes, while mice exposed to blue light took 16-19 minutes to fall asleep.

Levels of the stress hormone corticosterone were also measured, and found to increase significantly with exposure to all light colours.

To identify the role of melanopsin in this process, the tests were then performed on mice that are deficient in this particular pigment.

Interestingly, for these mice, the effects were reversed – blue light caused the mice to fall asleep in the shortest amount of time, while green light lost its rapid effect.

The mice lacking melanopsin also had far lower levels of corticosterone when exposed to light.

This finding, Peirson says, shows a link between the levels of stress or arousal, and the how the pigment melanopsin processes light: "There are different pathways from the eye to the brain - one directly regulating sleep and the other increasing arousal

“Melanopsin has a more complex role than previously thought, affecting both pathways. This is the first time that it has been shown to regulate adrenal stress responses.”

Of course, the nocturnal nature of mice must be taken into account when reviewing the findings.

Green light, he adds, may be expected to increase wakefulness in humans, and blue light would elevate our adrenal stress hormones.

The researchers hope the finding will contribute to a deeper understanding of the effects tablet and mobile phone screens have on our sleep patterns.

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