The science of sleep I: How the brain regulates sleep/wake cycles
Baby Sleep Study Co-Investigator
November 16, 2015 - 11:12
The rotation of the earth orbiting the sun – a single star – creates regular day and night cycles. Most organisms take advance of this situation by adapting their lifestyle to being active during one of these phases – nocturnal animals at night, diurnal animals during the day. Matching one’s sensory apparatus and behavior to the conditions in the environment provides a distinct survival advantage. Sleep and wake cycles are controlled by the activity of the pineal gland, which activates or shuts down regions in the brainstem that regulate waking itself. This process works through the release of Melatonin – a neurohormone. However, this raises a question: It is dark within the skull; how does the pineal gland – deep inside the brain – know whether it is night or day outside?
All light information enters the brain through the eyes. At the back of the eyes is the retina, a thin sheet of cells that converts light into electrical energy. Mammals use most of these electrical signals to form a visual image in the mind, allowing the organism to infer *what* is out there. In addition, a small number of cells in the retina functions to measure environmental light levels directly, simply telling the brain whether there is light in the outside world or not. These specialized cells, dubbed “intrinsically photosensitive retinal ganglion cells” (ipRGCs) send their signals to a small region just above where the optic nerves from the two eyes cross, the “suprachiasmatic nucleus” (SCN). It is the SCN that regulates the pineal gland, completing the circuit that regulates sleep and waking in mammals.
Importantly, ipRGCs do not treat all incoming light equally. They mostly respond to bright light. Furthermore, as light is an electromagnetic oscillation, lights have wavelengths. Whereas the interpretation of these lights by the brain is subject to idiosyncrasies, most humans perceive short wavelengths as blue, long wavelengths as red and a mix of all wavelengths together as white. The ipRGCs that report outside light levels to the brain seem to measure mostly the presence or absence of short-wavelength lights. Why does this matter? Because this system works well in the natural environment that humans evolved to grow up in. During daylight (the sky is blue), this system will report to the brain that it is indeed day outside, suppressing Melatonin release from the pineal gland, allowing the organism to go about their business. At night, it will indicate that it is dark and that it is time to sleep (see diagram).
In the modern world, screens and fluorescent lights put out bright light containing short wavelengths intense enough to activate the ipRGCs which will dutifully report to the brain that it is bright outside and that now is no time to sleep, suppressing the release of Melatonin. In essence, the brain is being tricked into believing it is day when it is night. In turn, the brain will try to maintain high levels of activation that disrupt the sleep cycle. But even in the modern world, there is something you can do to shield your ipRGCs from short wavelengths and give the brain the rest it needs and deserves. This is a topic that we will explore in detail in part 2 of our series on the science of sleep.