How Serotonin, the Happiness Hormone, Relates to Circadian Rhythms and How to Naturally Increase Its Levels

Preview: How is serotonin, the “happiness hormone,” related to circadian rhythms and how can you naturally increase its levels? This is what my colleague Tereza writes about in her article. It’s a scientific read, but all the more fascinating. Among other things, you’ll learn that serotonin not only affects mood, but also a range of bodily functions (including erection), that 90% of it is produced in the gut microbiome (no wonder it’s called the second brain), and how a lack of light influences its production. But most importantly, you’ll find out how you can help boost serotonin production yourself.

Light is the most important zeitgeber (time-giver) of our circadian rhythms, which are responsible for regulating various bodily functions, including the sleep-wake cycle. You’ve probably heard of the hormone of darkness, melatonin, which is closely linked to circadian rhythms. Its secretion from the pineal gland is controlled by signals from the eye through the optic nerve to the suprachiasmatic nucleus (SCN), the primary circadian pacemaker, the conductor that gets information on whether it’s day or night, or more specifically, light or dark. However, serotonin is talked about far less in connection with circadian rhythms, so we’ll shine a light on it in this article and take a closer look at its effects, as well as its relationship with melatonin and circadian rhythms in general.

Serotonin, the Happiness Hormone that Harmonizes Body and Soul

Serotonin, nicknamed the “happiness hormone,” is a very important neurotransmitter that transmits impulses not only in the central nervous system but also plays a key role at the periphery. It is most well-known for regulating mood, anxiety, and depression, appetite, and sleep; essentially, it’s involved in practically all human behavioral processes. Its production is not limited to neurons; more than 90% of serotonin is produced in the intestines by enterochromaffin (EC) cells, whose production is significantly influenced by our gut microbiome. Serotonin thus also affects gut motility, modulates immunity and inflammatory processes in the gut, and influences nutrient absorption. Its effects are far-reaching (see Fig. 1) because it binds to serotonin receptors, which are found throughout the body.

Picture 1: Effects of serotonin in the central nervous system (CNS) and beyond. SA node – sinoatrial node; AV node – atrioventricular node; HPA axis – hypothalamic-pituitary-adrenal axis. Information taken from source [1].

Serotonin as an Important Precursor to Melatonin

Without serotonin, there wouldn’t be enough melatonin. Serotonin is a key precursor in melatonin synthesis. This process is a two-step procedure and requires two important enzymes that mediate these reactions: serotonin N-acetyltransferase (SNAT) and hydroxyindole-O-methyltransferase (HIOMT), see Fig. 2. The activity of the SNAT enzyme is very sensitive to light, especially blue and azure light, which has been shown to reduce its activity in various animal species. Studies also confirm sensitivity to green and UV-A light. This is why melatonin synthesis is light-dependent [4], [5], [6], [7].

Picture 2: Schematic representation of melatonin synthesis. SNAT – serotonin N-acetyltransferase; HIOMT – hydroxyindole-O-methyltransferase.

Serotonin Levels Are Also Affected by Light

Research suggests that exposure to light affects serotonin levels, including related mood disorders. Reduced exposure to light also decreases the ability of the 1A receptor to bind serotonin, by as much as 30%. This can result in seasonal affective disorder (SAD), which is characterized by a drop in mood and depressive states, mainly during the winter months when there’s a lack of daylight. With reduced exposure, there’s also increased availability of the serotonin transporter, which ensures serotonin is reabsorbed from the synaptic cleft back into the neuron (see Fig. 3). In short, when there is insufficient light, serotonin cannot act for as long as it would under higher intensity. On the other hand, light therapy (phototherapy) can increase serotonin levels in the bloodstream and may also reduce serotonin transporter binding in brain areas that regulate mood. Given today’s modern lifestyle, where we spend most of our time indoors, it’s important to ensure adequate daylight and/or artificial light with enough intensity, mimicking sunlight with its spectrum [9], [10], [11].

Picture 3: Schematic representation of neural connection. Serotonin is a neurotransmitter that is released from synaptic vesicles from the pre-synaptic neuron into the synaptic cleft, where it can bind to serotonin receptors on the post-synaptic neuron, transmitting the signal to the next neuron. Serotonin transporters ensure the reuptake of serotonin from the synaptic cleft, ensuring its recycling.

Serotonin and Its Impact on Circadian Rhythms

There is a complex relationship between serotonin and circadian rhythms. A lack of serotonin leads to a deficiency of melatonin, which is crucial for the internal body clock that coordinates the organism’s adaptation to the time of day. On the other hand, serotonin directly modulates circadian rhythms by regulating the sensitivity of the suprachiasmatic nucleus (SCN) to light. The SCN is innervated by nerves with serotonergic signaling, and disruption of this signaling can potentially contribute to mood disorders and metabolic diseases. Disruption of the circadian system also impacts serotonergic signaling, meaning the relationship between serotonin and circadian rhythms is bidirectional [13], [14], [15], [16], [17].

Conclusion

For optimal serotonin levels, it’s important to get enough light exposure during the day, after which it transforms into melatonin at night. Make sure to expose yourself to light as much as possible during the day, and if you spend most of your time indoors with insufficient daylight, ensure sufficient light intensity from artificial sources, which should mimic sunlight and focus on light emission in the azure and red spectral areas to synchronize circadian rhythms and improve visual comfort. At night, maintain light hygiene to avoid disrupting your circadian rhythms, meaning use as little light as possible or none at all, and limit exposure to blue, azure, and green wavelengths.

Serotonin levels can also be influenced by regular exercise and dietary measures, particularly by consuming foods rich in tryptophan (a serotonin precursor) such as eggs, white and red meat, soy, bananas, etc., and folate (folic acid), which is found in high amounts in leafy vegetables (spinach, cabbage, lettuce, etc.). Proper light and sleep hygiene, combined with a balanced diet and regular exercise, can significantly improve your quality of life and mood during the day [18], [19], [20].

Mgr. Tereza Ulrichová, Spectrasol

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