Bright Light Before Bed Impairs Thinking the Next Day

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light at night impairs cognitive function

I’ve written many times about the effects of looking at your phone, TV, computer, etc before bed. It turns out that the intensity of light may also affect circadian rhythm and cognitive function – separately from sleep.

Here is a quick explanation of the effect of blue light at night if you need the background…

A recent study looked at the impact of exposure to bright light (1,000 lux) vs lower intensity light (150 lux) at night. The participants of the study (n=20) were healthy, young men in their 20’s. They were exposed to the lower intensity light (150 lux) between 8 pm and midnight for a couple of nights and then the bright lights for several nights. This was all done over the course of eight days in a sleep lab.

The 150 lux lighting would be similar to having some overhead lights on in a room — but ones that aren’t all that bright. In other words, the light was probably similar to what the men were accustomed to if they had some lights on at home between 8 pm and midnight.

The study participants then completed cognitive testing on the third morning to test their attention and decision making. Researchers also measured they frontal lobe activity during the cognitive testing with a near-infrared spectroscopy machine.

After that baseline testing, the participants again spent several nights in the sleep lab but were exposed to bright light (1,000 lux) each night between 8 pm and midnight. They were then subjected to another round of cognitive testing.

The results showed that bright light at night increased ‘commission errors’ in the decision making test. The researchers explain that this suggests an impaired inhibitory response, and the frontal lobe testing showed impaired right frontal lobe activation.

What does impaired inhibitory response mean?  Think of it as getting irrationally angry on your way to work in rush hour traffic and driving in ways that are a bit irresponsible. You know that cutting off that truck wasn’t the safest thing to do, but you did it anyway.

Why would brighter light at night cause differences in decision making the next morning? This study was done in a sleep lab, and the participants sleep patterns and time did not differ between the lower light vs bright light conditions.

The researchers theorize that the change in decision making could have been due to shifting the circadian rhythm.  Researchers in the past have shown that decision making changes based on time of day and circadian rhythm. I think this is something that we all kind of know already — I am much better at making decisions in the morning rather than evening, and it seems to have nothing to do with the amount of sleep, etc.


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