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Previous research has shown that blue light (short wavelength) captured by the retina before sleep has a detrimental effect on our sleep as it interferes with the production of melatonin (sleep hormone). However, the exact mechanisms driving these effects are still largely not understood. A recent study seems to contradict this theory, showing that while blue light interferes with hormone production, it doesn’t necessarily affect sleep quality and wakefulness upon waking. This suggests that blue light will have little effect on sleep in the long run. However, the subjects tested for the study were only exposed to light approximately 50 minutes prior to sleep.
The circadian rhythm, the biological clock that regulates our sleep/wake cycle, is controlled by a complex mechanism. It depends in part on a large number of environmental factors and governs many downstream biochemical events in our body, such as those that induce cognitive functions. Because of the complexity of the phenomenon, the biology of sleep is a very active area of research.
Exposure to light (and its various wavelengths) is one of the main factors affecting our sleep cycle. With today’s technological evolution, the vast majority of people spend at least six hours a day in front of a screen. Many studies then examined the effects of this exposure on our sleep and our overall health. The incriminated waves are, in particular, short waves (including blue ones), abundantly emitted by the screens of smartphones, computers and TVs.
A new study led by the University of Basel (Switzerland) aims to develop the theory that blue light suppresses the production of melatonin and thus interferes with the drowsiness necessary for good sleep. This type of light will mainly affect the light sensitive retinal ganglion cells (ipRGC) located between the cones and rods. Although the latter also capture light, ipRGCs are particularly sensitive to blue light and therefore may play an important role in circadian rhythm regulation.
The results, published in the Oxford Academic journal, suggest that in healthy young adults, sleepiness is more likely to be caused by sleep stress, i.e. the accumulation of daily fatigue, rather than exposure to blue light.
Study limited to 29 participants
For their study, the researchers recruited 29 apparently healthy people with an average age of 23 and a history of relatively healthy sleep (average bedtime at 11:00 pm). During the tests, they were exposed to light from a screen that was turned off about 50 minutes before normal bedtime. A week later, participants were exposed to a different type of light before bed.
According to participants’ reports, the two light sources seemed almost identical to them, and their sleep EEGs seemed to confirm no difference. However, one of them was highly concentrated in blue light. To measure melatonin levels, the researchers took saliva samples every 30 minutes for five hours before bed and also in the morning after waking up. Participants were also asked about the quality of their sleep and how alert they were when they woke up.
Participants then reported no light problems with high blue light levels despite an average 14% drop in melatonin levels. This suggests that sleep pressure dominated blue light effects. These results also indicate that the effects of blue light on sleep quality may be caused by other cell types than ipRGC.
However, it should be borne in mind that the blue light exposure of the candidates was interrupted long before sleep in order to be able to interview them. Effects would probably be different if exposed closer to sleep. Exposure to screens in the evening will thus only affect sleep, probably only after a certain duration and intensity before sleep. According to experts, the new study will in no way refute the old theories about blue light, but rather complement them.
In addition, these experiments did not take into account many other factors that affect the sleep cycle, including homeostatic sleep drive, mental state, physical health, environment, etc.