Light supplies the energy of life in many ways. When light hits the extremely complex molecular factories for photosynthesis, it triggers an electron that moves to the processing center by quantum superposition; this electron stimulates complex chemical cycles that make energy particles—ATP—as well as carbon molecules like sugar. Light bounces off objects landing in the eye’s retina to form images in the brain, allowing sight. Light, also, provides the stimulus for cellular clocks, which regulate all metabolic processes, including wakefulness and sleep. Light stimulates skin cells to start the process of making vitamin D, vital for all cells including the brain.
Light has long been a metaphor for advanced mental processes—enlightenment. In this regard, images of light have been used as methods of meditation to arrive at relaxation and expanded mental states. Now, research shows that special light sensitive cells in the eye are not used for vision, but rather, send neuronal fibers through complex circuits directly into higher brain centers of learning and emotion. This research shows that light is critical for learning, cognition and positive mood. In fact, sunlight stimulates chemicals that can make us “high” like a drug and can be addictive, including withdrawal. These unusual non-vision retinal cells have, also, been shown to alter regular vision in very subtle ways. It is now clear that light in the brain has many critical effects.
Retinal Cells – Rods, Cones and Non Visual Retinal Ganglion Cells
The rods and cones in the retina receive photons of light and this energy triggers an electrical signal to the retinal ganglion in the brain. In the last ten years another system of cells was discovered that, also, responds to light but in a totally different way and with different effects. This was confirmed by finding cells in the blind that regulate melatonin, the secreted hormone that notes the time for sleep. Later animals with no rods or cones detected light by this pathway.
Special proteins (opsins) are photoreceptors that absorb light particles (photons) and alter the membrane’s electrical potential, which triggers a signal into the brain.
Humans have 120 million rods and 6 million cones for vision. Rods are very sensitive and can react to a small number of photons and work in very low light without colors (there are more in animals that live at night). The photoreceptor protein in rods is called rhodopsin. Cones need bright light and have three different types of photoreceptor proteins for different colors. The photoreceptor protein in cones is called photopsin.
The non-visual ganglion cells have a different photo receptor protein called melanopsin that is similar to that of plants and creatures early in evolution. Approximately 5% of all the photo receptor cells are the non visual type that sits in front of the rods and cones in a different layer.
Intrinsically Photosensitive Retinal Ganglion Cells – ipRGCs
Light stimulates cells in the retina of the eye that are not part of the visual system. These cells have an unusual name—intrinsically photosensitive retinal ganglion cells, or ipRGCs. ipRGCs are the cells that directly stimulate behaviors. They are critical for human life, but have nothing to do with seeing. Recently, it was discovered that there are at least five different types of ipRGCs, each with complex wiring to many different parts of the brain related to cognition, mood and behavior. ipRGCs have different shaped neurons and electrical patterns. It is possible that each has different effects on emotions and cognition, but there is definitely some overlap. The many complex circuits are not yet fully understood.
The normal use of sight through rods and cones to perceive surroundings have little role in the effects of light on mood, cognition and sleep.
ipRGCs work independent of vision and affect sleep, cognition and mood. One type of ipRGCs, called M1, sends axons to the center of the body’s circadian clock. This clock center is the suprachiasmatic nucleus (SCN).
The various subtypes have connections with many different brain regions:
- Subparaventricular zone and intergeniculate leaflet – related to circadian rhythms.
- Ventrolateral preoptic area (VLPO) and lateral hypothalamus – related to sleep and alertness
- Medial amygdala and lateral habenula – related to mood.
- Cortex – related to cognition
Some of the five types have circuits to the pre optic and lateral hypothalamic centers related to hormones, the VLPO, which induces sleep, and the ventral suparaventricular zone, which controls general level of activity.
There are direct connections from ipRGCs with the limbic emotional centers, especially the lateral habenula and the medial amygdala.
Direct and Indirect Brain Circuits Respond to Light
The direct effects of light are circuits starting with igRGCs that directly impact on brain centers related to sleep, mood, and cognition. The indirect pathways affect the circadian clock first, which has vast connections to all physiological functions. These two complex systems interact in many ways that are not fully understood.
Light, through the direct and indirect systems, provides synchronization of the daily clock for metabolic processes. They allow our bodies to understand the seasons and regulates sleep and wakefulness, as well learning and mood. Activity and rest are regulated in different ways. Organisms have become accustomed to the 24-hour cycle and even hidden from light the internal clock will run in 24-hour cycles.
Circadian Clock – The Indirect System
Light has dramatic effects through the circadian clock—the indirect system of light influence. There is one central clock (SCN in the hypothalamus) and many peripheral clocks in bodily tissues that interact with the environment and the central clock.
Peripheral clocks are in esophagus, lungs, liver, pancreas, spleen, thymus, skin, and prostate. Liver cells respond to food, not light. Many parts of the brain, including areas critical for mood and cognition have their own peripheral clocks that interact with the central system. The central clock runs by itself from internal mechanisms, but is adjusted by natural light. Peripheral clocks affect metabolic functions related to complex human activity.
Altered light can disrupt the connection of the central clock with the peripheral clocks leading to depression, poor sleep, and cognitive impairment. The peripheral clocks in many brain regions have complex interactions with the SCN. Emotional brain regions such as the amygdala and the lateral habenula have clocks that are related to depression. The lateral habenula connects with critical brain regions for emotion including the VTA, raphe, limbic and hypothalamic and brainstem.
The circadian clock drives hormonal changes, sleep wake cycle, and metabolic cycles. Animals become accustomed to performing behaviors in the appropriate times. Because of the clock, the organism accumulates sleep need during awake periods and then this metabolic pressure decreases during sleep. This sleep drive that builds up, along with circadian pathways determines how long we sleep. This sleep drive is greatly affected directly by light, as well as the initiation of sleep.
Alterations in the amount of light, affect sleep drive and the amount of sleep. Those who work at night, during short days of winter, and travel that alters the amount of sun can have many health related effects, including depression and altered cognition.
External light can reset some aspects of the clock, including the cycle of core body temperature, brain waves, and hormone production. In the migration of butterflies, the clock determines both the sun compass for bright days and magnetic compass for cloudy days.
The SCN stimulates the pineal to secretes melatonin and affect levels of cortisol and testosterone.
A summary of the effects of circadian clock on metabolism:
- Early morning: stops melatonin, increases blood pressure, stimulates bowel movement, increases testosterone, most alert, best coordination
- Afternoon – fastest reaction time, best cardiovascular efficiency, most muscle strength,
- Evening – highest blood pressure and body temperature
- Night – suppress bowel movements, more melatonin, sleep
Alterations in Mood
The direct path sends its own signals to regions that affects mood and cognition, but has interplay with the circadian clocks. Light can, also, directly alter mood without affecting the circadian mechanism, and without sleep deprivation.
Depression, Sleep and Light
Depression often causes very poor sleep, usually too little, sometimes too much (especially in bipolar depression). In bipolar disease very depressed patients improve if they are deprived of sleep, but if they continue to lose sleep, they can become manic. Manic patients usually do not sleep and this fuels the mania. Manic patients have much less sleep and don’t appear to need sleep, staying up for days with racing thoughts and activities.
Mood disorders, also, stimulate imbalances of hormones, most notably cortisol and melatonin. In depression, there is less slow wave deep sleep. Also, dreaming occurs sooner with more episodes of dreaming.
Sleep deprivation clearly causes problems with cognition—poor learning and memory and decreased concentration. There is recent research connecting less sleep to onset of dementia. Sleep deprivation for a brief time alleviates depression for that day, but if long lasting, sleep deprivation causes mood disorders.
Environments with altered light (night shift, winter) disrupt the circadian rhythms and sleep, which can cause mood problems. Seasonal affective disorder (SAD) occurs as fall and winter have much less light and is more prominent in latitudes where the seasonal changes in the length of day are most extreme.
Melatonin is secreted by the pineal gland at the time when sleep should start. When light is increased at night, people become more alert and they decrease the amount of melatonin. Both SAD and people without depression can have altered melatonin cycles, so this is not the cause of SAD. Melatonin is altered directly by light. SAD patients have altered sensitivities in the retinal cells.
Winter, Night Shift and Circadian Phase Shift
In bipolar disorder, less light in autumn often stimulates depression lasting through the winter. In the spring, mania is more common. Bright lights have, also, helped some bipolar patients. Too much light treatment can stimulate mania.
With rapid travel by jet airplanes, it is now much easier to confuse the circadian rhythm especially travelling between northern and southern hemispheres.
Cortical steroids that are altered in depression are, also, involved in resetting the circadian phase with jet lag. Cortisol is especially altered in travel between the northern and southern hemispheres. One study showed that travelers who frequently go between these hemispheres—crossing the meridian 50 trips a year for several years—had cognitive changes from chronically disrupting these hormones, including frequent high levels of cortisol.
Workers at night have extreme artificial light that disrupts the circadian and sleep wake cycle. It is much more difficult to sleep during the day and stay up all night, causing very poor sleep.
Light and Learning
Short periods of light such as occur in winter, cause depression like behavior and learning difficulties in experiments with animals. The hippocampus cellular structures are altered as well as less long-term potentiation neuroplasticity.
With shifts that resemble jet lag, the circadian system’s connection with other bodily tissues that normally respond to the clock are altered. It alters genetic networks in the central clock. They, also, show increased learning difficulties if it continues. Just one episode affected conditioned fear responses.
Keeping the circadian clock in order appears to be important for learning. Animal experiments show:
- If light is pulsed or phase delayed, hamsters showed hippocampal learning problems. The same results occurred with a day structured for only 20 hours, but the brain alterations were different in learning centers, including PFC decrease in dendrites, as if lesions in this region.
- With constant light, a different set of problems occurs including poor spatial learning, memory and depression like behavior. If they were provided a place to escape the constant light they avoided depression.
- Dim light throughout the night caused depression like behaviors. Different species of rats had different effects. A diurnal species had decrease in spatial learning and less dendrites in the learning centers of hippocampus. Hamsters had smaller dendrites and less movement.
- If the central clock (SCN) was eliminated by genetic or surgical means, rats became less depressed and more manic. This is a direct effect of light not related to circadian mechanisms. It is not clear if this is from direct stimulation of the ipRGCs
The normal activity is altered by either direct effects on ipRGCs on the brain or by changing the clock.
Blue Light in Humans
Bright lights can cause increased alertness and vigilance by stimulating specific cortex and thalamus regions. As soon as the light stops, the effect stops. Blue light, in the 480nm wavelength, activates melanopsin best in humans and directly stimulates brain regions for attention, alertness and emotions. This helps learning and mood. This blue light, also, affects the blind in the same way through the ipRGCs.
In patients with seasonal affective disorder, the blue light affects the posterior hypothalamus, as well as the locus coeruleus (the center of norepinephrine) and the dorsal raphe nucleus (the center of serotonin) the well-known neurotransmitters that are affected by antidepressants.
With circadian clock, light helps or hurts depending upon the time of day or night. Light at night caused depression effects. Bright light therapy has been useful for SAD but also for non-seasonal depression and bipolar disorder and helps the effect of antidepressants.
Summary of Resent Research Into Light’s Effects in the Brain
Research is demonstrating an increasingly wide range of specific effects of light in the brain, such as light creating molecules of addiction, “highs” and withdrawal. The following is some of the additional recent research:
- A very new study shows that ipRGCs not only have behavioral connections but also have unusual effects on vision, especially determining high contrast sensitivity
- ipRGCs go to the olivary nucleus determining the pupil light reflex
- Ultra violet (UV) light elevates beta-endorphin, causing analgesia reversible by opiod blockers. UV light leads to dependency and addiction behavior (tanning). Epidermal keratinocytes synthesize POMC, which becomes melanocyte stimulating hormone for tanning. POMC also produces β-endorphin (another peptide) in skin and then increases blood levels. After chronic UV exposure, there is a withdrawal.
- Light stimulates alertness and cognition, increasing wakefulness and performance. Long wavelength light enhances executive brain function by melanopsin mechanisms. Melanopsin is similar to plant receptors. Prior light can increase the next response to light, called photic memory. This occurs with melanopsin and plant photo pigments as well. Previous light history effect is an adaptation to previous exposure to certain light conditions in causing increased performance.
- Light produces vitamin D in the liver and kidneys from chemicals produced in the skin from UV light (cholecalciferol is D3 converted in liver to calcidiol measured Vitamin D in blood. Kidneys can make calcitriol the most powerful). There are vitamin D receptors in the cortex and hippocampus
- Sunlight increases serotonin (serotonin might have developed in evolution attempting to respond to the environment).
- Sunlight boosts dopamine, associated with pleasure, reward and addiction.
- Light suppresses melatonin causing more wakefulness.
Light in the Brain
Light is not just a metaphor for brain effects and enlightenment. In fact, it is essential for learning, cognition and mood. There are a wide range of direct and indirect effects of light on all aspects of physiology including sleep and wakefulness, hormones, mood, and the ability to learn. Retinal ganglion cells that are not involved primarily in vision have a wide range of direct effects on circuits that affect critical regions of the brain related to learning and mood. There are at least five distinct types of cells and circuits but these are just being discovered and there could be more. With the two interacting complex systems of direct and indirect influence of light on the brain, there is much more to be learned.