Scientists have reported that subjects immersed in long wavelength orange light showed enhanced brain activity during tests conducted an hour later relative to those exposed to short wavelength blue light. This backs up the long-running, but difficult to prove, hypothesis of "photic memory".
Published recently in the journal Proceedings of the National Academy of Sciences (or PNAS — the best place to publish if you have the sense of humor of a 12 year old boy), researchers had 16 test subjects perform simple cognitive tests inside an MRI scanner while they were illuminated with either orange, green or blue light. The subjects were then blindfolded for over an hour to let the rods and cones in their eyes completely readapt to darkness. They then repeated the tests while again being monitored by MRI.
What they found from the second MRI analysis was that, even after over an hour of darkness, the orange light had a substantially stronger impact on the prefrontal cortex of the particpants — the part of the brain responsible for, among other things, higher-order cognitive function.
One explanation for these results is the hypothesis of "photic memory". Not too many years ago, scientists discovered a whole new set of photoreceptors in the eye which are not involved in image forming. These receptors express a light-sensitive retinal protein called melanopsin, which is known to be responsible for things like sleep cycles. It is also thought to affect cognition and this is the very first human study to give some credence to that idea.
See, the rod and cones in our eyes readapt to darkness after about 30 minutes, whereas melanopsin takes a much longer time. The authors therefore suggest that the MRI anomalies observed after 70 minutes blindfolded can only be due to melanopsin-mediated influences on the brain.
It's still early days for this hypothesis, but you may want to consider orange lights for your next last minute cram session.
You can check out the full article, Photic memory for executive brain responses, here.