"Turn on the light, so you don't ruin your eyes!" almost every grandmother from Aš to Zlín used to command at some point. But even grandmothers are sometimes wrong – and dim light does not ruin the eyes. Nevertheless, proper lighting is useful for work: recognizing letters is more strenuous in poor light, so a person's eyes get tired sooner. And light also affects the activity of the brain and, indirectly, the whole body. Its lack suggests to the organism that it is time to sleep, thereby reducing attention and increasing fatigue.
Behavioral engineers speak of a "spectral diet": they study the properties of light that we receive from different directions during the day and correlate them with behavior. Indeed, information about light falling on the retina does not only flow from the eye to the visual areas, which derive data about the space around us. Signals about photons landing on eye cells also travel to centers that orient us in time and, based on the time of day and season, decide how much energy we want to consume – and regulate the activity of the entire body, including the brain, accordingly.
"In humans, bright light stimulates emotional well-being and cognitive functioning," the authors summarize the conclusions of last year's review study. This is by no means specific to humans; similar conclusions are brought by the study of almost all diurnal animals – and for nocturnal ones, the exact opposite applies. In the course of evolution, we have learned that gathering, hunting, and any other activities are more profitable when conducted during the day, and especially in summer. Bright daylight, specific particularly for its high proportion of radiation that we perceive as blue, therefore awakens us to action.
On the retina, the pigment melanopsin responds to blue light; while it does not enable us to see, it recognizes a bright day. This also led to its description long before scientists actually found it in 1999: already in the middle of the century, they discovered that even mice genetically deprived of retinal cones and rods (and thus of sight) respond to light. Neurons then send the signal about the activation of melanopsin to the suprachiasmatic nuclei (the long word in the name is not arbitrary: it comes from the prefix "supra" indicating an upward direction and the root "chiasma", used in Latin for a crossing – because these clusters of neurons are located roughly a third of the way into the brain from the forehead backwards, precisely above the chiasm of the optic nerves from both eyes, which bring that information about light).
In the suprachiasmatic nucleus, our internal clocks tick, measuring time through gene transcription – and it is precisely its speed that light can influence if our "internal" clocks are not in harmony with the external world. From here, hormonal signals also flow to the "genetic clocks" of individual organs in the body, which synchronize our biological rhythms.
For the brain, it is fundamental that light affects the production of two key neuromodulators: melatonin and hypocretin. The release of melatonin, which prepares us for sleep, is suppressed, while the increased production of hypocretin stimulates us to activity. Hypocretin neurons increase tension, encourage us to move, and amplify our motivation to achieve goals. Their importance is best illustrated by what happens in the event of their loss: major damage to these centers leads to narcolepsy, a condition of constantly falling asleep in the middle of any activity.
Hypocretin centers namely "awaken to life" the entire brain:
- the thalamus, which transmits sensory inputs into the brain – so the organism perceives better.
- the release of serotonin, the level of which most antidepressants try to increase – and besides managing stress, we also need it for learning and memory.
- the release of dopamine, which signals the possibility of achieving a reward in the brain – thus motivating the organism to try to reach a goal.
- the release of noradrenaline, the supply of which we need for managing attention and information processing – so it helps direct attention to what is important.
- histamine, which increases brain activity, thus aiding the performance of cognitive functions.
Light is, of course, by far not the only thing that affects our brain – however, thanks to a combination of these mechanisms, we can use it, for example, to increase the alertness of pilots and flight attendants. In other studies, targeted light exposure improved working memory or reduced reaction time.
The potential of light, however, reaches even further. From an engineering standpoint, light is essential for safety, for instance – research is currently underway into the connection between the perception of light and temperature. Lighting namely affects our sense of thermal comfort, so appropriate illumination can contribute to energy savings for heating. We won't notice such a decrease in a family budget – but from a population perspective, it is well worth some thought. Medicine, on the other hand, utilizes the effects of light in treating insomnia and depressive disorders, and is testing lamps and light glasses even for neurodegenerative diseases in old age, particularly Parkinson's disease. The latter is linked to light via a dopamine mechanism.
However, even more interesting applications are emerging. In Japan, "light barriers" help prevent suicides. The study of light effects finds utilization in sports, where special lamps help elite athletes adjust their biorhythms to the time zone they are currently competing in, so they can deliver their best performance. And for non-athletes, intense research is being conducted on the relationship between lighting and obesity. As far apart as the two may seem, daily fluctuations of light affect almost all bodily rhythms, including insulin release and other processes related to hunger, digestion, and energy consumption. Artificial lighting is thus likely contributing to the societal growth of obesity – to the extent that, according to pilot studies, it is worth including light therapy in weight loss programs.
The study of the biological effects of light has thus grown into quite a comprehensive science. For an ordinary person, however, it is enough to follow three quite simple rules. First, do not disrupt evening and night rest with the light of LED lamps and displays. Falling asleep is of course possible even with eyes fixed on a phone, but dimming and moving the monitor or mobile away, or switching to night mode, will support sleep. Second, expose yourself to light during the day. Daylight helps the organism distinguish between day and night, and its lack in sensitive people contributes to the risk of depression – so fundamentally that too little light may cause some depressions, although other factors always play a role. And third, adapt your life rhythm to the external world – and maintain its regularity. Of course, it is not necessary to get up with the dawn, but the body simply functions better if the signals from the external environment and the learned rhythm synchronize. Although the specific timing of sleep is subjective, working at night and sleeping during the day is not beneficial for a person. Well, and if you work at the same time, it is worth turning on the light for your work.
