Marie Dumont, PhD University of Montreal
Light suppresses the production of melatonin, and light causes a shift in the sleep/wake cycle. Are these two separate processes? If so, which process makes light therapy work?
Both processes use the same system in our retinas to process the exposure to light. This system is called “non-visual“ because it is different from the system used to see the objects in our environment.
Specialized receptors in our eyes—ganglion cells that contain melanopsin (not the same thing as melatonin!)—respond to the presence of light, and send the message that light is present to our main biological clock. This clock is located in the brain—or more precisely, in the suprachiasmatic nuclei of the hypothalamus.
The Timing of Light
Specialized cells in the biological clock analyze those messages and may take action depending of the time of day. If the light signal is received during daylight hours, the clock will think it is normal to have light at that time, and will do nothing.
If light is received in the late evening, however, the clock will push all biological rhythms later (initiating a “phase delay,” including the sleep-wake cycle.
If light is received at the end of the night or very early the morning, the clock will understand that the environmental day has started sooner than expected, and will initiate a phase advance to pull all rhythms toward an earlier time.
Importantly, if the light signal is received during night, the clock will interpret the light signal to mean that it is daytime in the environment. It will then initiate changes within its molecular machinery to adjust the timing of the biological day to the environmental day. This is what we call a phase shift.
The biological clock also controls the timing of the secretion of the hormone melatonin. Melatonin is produced by the pineal gland, also located in the brain, with a privileged pathway to communicate with the biological clock.
Melatonin is a hormone that is normally secreted at night, and the presence of melatonin in the body means that it is the biological night. When the biological clock receives light during the night, it urgently sends a signal to the pineal gland to stop producing melatonin so the body will understand that it is not night anymore.
In humans, melatonin helps you feel sleepy at night. When melatonin production is stopped by nighttime light exposure, it makes you less sleepy, which could be useful if you have to stay awake at night.
Both phase-shifting and the end of melatonin production depend on the non-visual perception of light by the biological clock during the night. However, while the shift in the sleep-wake cycle (and other circadian rhythms) derives from molecular mechanisms within the clock, the suppression of melatonin production occurs in the pineal gland in response to an urgent signal received from the clock.