Melatonin and sleep
Melatonin is a hormone secreted by the pineal gland. It is produced and released during certain times of the day with a daily rhythm (circadian rhythm), with levels peaking at night. Exposure to light serves as a regulating mechanism telling one’s body when and how much to release. During the day, production is stopped as we are exposed to light.
Since light suppresses melatonin production, looking at lit screens at night leads to problems with sleep and possible health problems as a result. While light of any kind suppresses melatonin production, blue light suppresses it the most. Its effects last twice as long as green light, for example. This may be because blue light in the visible spectrum is the closest to the ultraviolet region, which constitutes for 47% of light emitted by the sunlight. Since the human body has experienced light solely from the sun much longer than, say, smartphones, it makes sense for our bodies to be more responsive to blue light.
One could argue that red light would suppress just as much melatonin, since it is closer to the infrared region and infrared light also constitutes 47% of the sun’s emission spectrum. While this is a good point, studies have shown that red light does not suppress melatonin as profoundly as blue light. The brightness of light plays a role in melatonin suppression as well as the color (more accurately, the frequency or wavelength).
Besides light, stress plays a role in sleep some may not expect. Cortisol is the hormone produced that tells the body how to react when presented with stress. Cortisol operates on the opposite cycle of melatonin, so increased levels of cortisol at irregular times will interfere with melatonin’s cycle and thus result in lower quality sleep. While melatonin plays an important role in sleep, it is also linked to puberty and menstruation.