Not only light improves the feelings related to depression, dementia or sleeplessness, it also seems to affect our ability to think and cope with stress. In 2019, a respected journal Hormones and Behavior published a study summing up the current knowledge on the effect of light on emotions, along with further sophisticated experiments.

Repeatedly falling asleep and waking up again, no matter about the time of the day or night. Sounds terrible? This disorder is called narcolepsy and is caused by loss of neurons producing substance called…

Meet hypocretin and orexin, the molecule with two names

…sorry, I needed a little nap. The substance lacking in narcolepsy is called hypocretin or orexin: being simultaneously discovered by two independent teams, it was left with two names. I hope it is not too confusing for that poor little chemical.

The basic role of hypocretin is to get you moving. Literally: hypocretin directly promotes behavioural arousal and locomotion. Also, it stimulates the reward paths in the brain, so that they can motivate you to pursue some meaningful activity. And it facilitates thinking, memory and cognitive functions in general. And what is vital: hypocretin production is inhibited at night, so the whole promotion of arousal only works when you stay awake. And especially when you are hungry, to make you search for food.

With properly working circadian system, your brain knows when to sleep and when to work. During the day, hypocretin is suppressing production of melatonin, the molecule of sleepiness.

As we know from the previous article, it is light what suppresses release of melatonin during the day, so this function of hypocretin may be seen useless. But it is not, because our ancestors needed to keep the rhythm going even when they were hiding in a cave – and we certainly need to say day from night when staying indoors, too. Plus, the hypocretin mechanism is not merely a backup, but also a little thingy making the arrival of evening sleepiness smoother.

Once evening comes and the light does not keep the melatonin producing cells off, hypocretin is not strong enough to keep them on leash. Despite the hypocretin-caused inhibition, gradually, bit by bit, you start producing melatonin. Importantly, the grudge between melatonin and hypocretin is mutual, so not only hypocretin inhibits melatonin production, but melatonin also switches hypocretin production off. Therefore, once the light goes off, the gradual melatonin rise slowly starts switching off the hypocretin-producing neurons in lateral hypothalamus. And the less hypocretin is produced, the more melatonin can be released.

Can you see the spiral here? Steady melatonin expansion is further limiting the hypocretin levels, which in turn unbreak more melatonin, until you place your tired head on your pillow.

This is, however, only one of many feedback loops. You already know that hypocretin is released when you are awake and that it makes you physically moving. But that is only half of the story, with the second half being that physical activity like moving around or moving at all promotes release of many more molecules that further promote wakefulness.

Lack of hypocretin therefore not only causes attenuation of motivation and behaviour – but indirectly also affects the ability to stay awake. And together with many other substances, this may lead to depression.

Serotonin, dopamine and other internal antidepressants

Several of these feedback loops involve substances crucial for putting ourselves together when it comes to a losing streak. Related to depression, you might have heard of serotonin: the substance most today’s antidepressants target. Although we have not fully understood the role of serotonin regulation in depression, most of the evidence suggests that depressed people lack this chemical. Therefore, antidepressants are trying to increase its levels in the brain.

Increasing serotonin levels is not as easy as it sounds, because with our brain being protected from most substances in the blood, if you were to eat serotonin in pills, it would never reach your brain (but would harm you elsewhere). Therefore, instead of adding more serotonin, the most common pills on depression block the chemical processes that remove molecules of serotonin from the synapses between neurons. Or at least that is what we think they do: unfortunately, there is no recorder inside the brain to show us what is happening there in detail.

Surprisingly, phototherapy works in a very similar fashion. Through its effects on melatonin and hypocretin, it stimulates production of serotonin. And indeed, antidepressants and light therapy work well together! Besides serotonin, light also stimulates release of dopamine and other chemicals, that make phototherapy likely to be beneficial for patients with Parkinson’s disease and other dopaminergic problems.

Similarly, it was also likely that by increasing levels of other neurotransmitters, light therapy might improve our attention and cognitive processes in general. And that is exactly what the study of 2019 confirmed.

First, the authors gathered evidence that bright light has positive effect on anxiety and depression. But that is not all! They also confirmed that phototherapy improves cognitive processes, problems related to dementias and internal bodily regulations: capacities to cope with stress, digestion or recovery after injury.

“Evidence from the literature on both clinical and non-clinical populations has firmly established that environmental lighting condition is an important modulator of mood and cognition in humans,” the authors conclude.

Light made mice smarter, happier and less susceptible to stress

With their research, the scientists wanted to shed light on the mechanism of these beneficial effects. As it would be unethical to lock humans in labs, they use a unique rodent species, which, like humans and unlike its biological relatives, is active in the day and sleeps during night: the Nile grass rat.

They kept the mice in a laboratory, mimicking either dim winter light or bright and long summer days. Finally, after four weeks, the testing day came, with puzzles and mazes. Those animals who did not have enough light in the previous weeks, were acting like depressed and anxious humans: moving more slowly, hiding more often and had lower preference for sweet rewards. Moreover, it took them longer to learn how to get through the maze: a clear sign that their learning abilities were impaired.

But it was not only feeling and thinking affected by the lack of light, but also their inner stress mechanism. Chemical analysis of stress hormone cortisol in their saliva revealed that when exposed to the same stressor, those lacking light reacted by higher cortisol release. In other words, lack of light made the stress system activate more strongly in reaction to the same stimulus.

How can light affect our cognition?

The authors suggest key role of hypocretin – and they support their claims with strong evidence:

  1. When hypocretin neurons were scrutinized, differences were found between the mice held under bright light and dim light conditions.
  2. Inhibiting hypocretin receptors in the animals living under the bright condition resulted in similar depressive and anxious behaviour as in those lacking strong light.
  3. The lateral hypothalamus that produces hypocretin receives input directly from retina. And we know that hypocretin release is strongly influenced by melatonin that crucially depends on light.

Hypocretin, in turn, is known to play a vital role in many cognitive and emotional processes and related problems.

  1. Depressed or anxious people have less hypocretin in the cerebrospinal fluid. And probably also less hypocretin-producing neurons. Unfortunately, this was so far only proven on animals, because we can hardly open the scull of a depressed human just to check the brain tissue.
  2. Hypocretin dysregulation is the main cause of narcolepsy and possibly other problems with sleep and arousal. Naturally, it also affects cognitive capabilities. In humans, hypocretin is related to dementia and cognitive decline following stroke, and in animals it was proven to affect learning.
  3. Hypocretin is well known to stimulate production of serotonin, noradrenaline and dopamine: neurotransmitters that are targeted by antidepressant medication. We already covered this in depth in our article linking the light and depression.

“Our work has revealed that chronic dim daylight intensity results in higher depression- and anxiety-like behaviors, as well as impaired spatial learning and memory. Furthermore, we have found that hypothalamic orexin is a mediator of these effects,” the authors summarize.

Yan, Lonstein, Nunez (2019). Light as a modulator of emotion and cognition: Lessons learned from studying a diurnal rodent. Hormones and Behavior, Volume 111, May 2019, Pages 78-86.

Understanding the link between light, circadian rhythms and our internal activation, you can check more comprehensive article exploring the mutual interaction between depression and light perception. The next article from the Science Behind series presents recent research findings that it is not only light what affects our mood, but that our mood also influences how much light our retinas perceive: